Physics 105 - How Things
Work - Fall, 1999
Final Examination
Given Wednesday, December 15, 1999, from 9:00 AM to 12:00 Noon
PART I: MULTIPLE CHOICE QUESTIONS
Please mark the correct answer for each question on the bubble sheet. Fill in the dot completely with #2 pencil. Part I is worth 67% of the grade on this examination.
Problem 1:
You are riding a skateboard down a gradual incline. In order to waste as little energy as possible while still maintaining control over your direction of motion, the contact between the sidewalk and the wheels should experience
(A) only static friction.
(B) no friction.
(C) both static and sliding friction.
(D) only sliding friction.
Answer: (A) only static friction.
Why: Static friction wastes no energy while sliding friction does. To maintain control, you need sideways forces and those only come from friction, so you need static friction.
Problem 2:
In Charlottesville, an egg left in boiling water for 10 minutes becomes a firm, "hard boiled" egg. On the top of Mount Everest, with a height of 29,029 feet, an egg left in boiling water remains soft indefinitely. The egg fails to cook on Mount Everest because
(A) water boils at low temperature when the air pressure is very low.
(B) water boils at low temperature when gravity is weak.
(C) water has poor thermal conductivity when its pressure is very low.
(D) an egg cannot get very hot when gravity is weak.
Answer: (A) water boils at low temperature when the air pressure is very low.
Why: When the pressure is very low, steam bubbles that form inside water don't need much pressure in order to grow by evaporation. The air pressure outside isn't enough to crush the bubbles, even at relatively low temperatures. On Mount Everest, bubbles that form in 70° C water are stable and grow via evaporation. Water boils at only 70° C so that boiling water isn't hot enough to cook an egg.
Problem 3:
Your car has a flat tire on a deserted road and you can't find the jack. Fortunately, you have a box of enormous plastic bags and a portable leaf blower. You put one of the bags under the car and use the blower to inflate it. The bag easily lifts the corner of the car so that the damaged tire is off the ground. You change the tire and continue on your trip. While the pressure inside the bag never reached more than twice atmospheric pressure, the bag was able to exert a tremendous upward force on the car because
(A) the increased pressure was exerted on a large surface area underneath the car.
(B) inflating the bag made it spherical and spherical objects are stronger than any other shape.
(C) air from the leaf blower was traveling fast and fast-moving air exerts lift forces on anything it encounters.
(D) air from the leaf blower was traveling fast and fast-moving air exerts drag forces on anything it encounters.
Answer: (A) the increased pressure was exerted on a large surface area underneath the car.
Why: It doesn't take a large pressure imbalance to produce a large force. However, a modest pressure imbalance must act across a large region of surface for that resulting force to be large.
Problem 4:
You have just landed a new job with a corner office near the top of a high-rise building. You soon find out why this office was available; it is on the sunny side of the building and becomes uncomfortably hot on sunny days even when it is quite cool outside. Although your college physics course is a distant memory you have an idea of how to solve the problem. You cover the inside of the windows with aluminum foil. This results in
(A) a reduction of radiative heat transfer since the shiny aluminum foil has a low emissivity.
(B) a reduction of convective heat transfer.
(C) a dark room that is just as hot as before you covered the windows.
(D) a reduction of conductive heat transfer from the cool outside air to the warm room inside.
Answer: (A) a reduction of radiative heat transfer since the shiny aluminum foil has a low emissivity.
Why: Aluminum is a poor absorber and emitter of thermal radiation. It has a low emissivity (coupling to radiation) and reflects that radiation rather than absorbing it. The sun's thermal radiation bounces off the aluminum foil and little of its energy reaches the inside air.
Problem 5:
When a log is floating on water, much of the log is above the water and is actually surrounded by air. If that surrounding air were to suddenly disappear, the log would
(A) move downward slightly and float somewhat lower (deeper) in the water.
(B) float at the same height as before the air left.
(C) sink to the bottom of the water.
(D) move upward slightly and float somewhat higher (less deep) in the water.
Answer: (B) move downward slightly and float somewhat lower (deeper) in the water.
Why: The magnitude of the buoyant force on the log is equal to the weight of everything it displaces: water and air. The presence of air at the beginning adds to the buoyant force on the log. When that air is removed, the water must provide all the buoyant force and so the log slips slightly deeper in the water before being fully supported. (Note that removing the air would cause the water to begin boiling. Nonetheless, the log would slip deeper into this now boiling water before it would be properly supported)
Problem 6:
You are juggling grapefruits in your hands and are about to toss one far above your head. After it leaves your hand, the grapefruit will experience
(A) the upward force of its weight as it rises. Once the grapefruit reaches its maximum height, it will begin to experience the downward force of its weight.
(B) an upward force as it rises. This upward force will gradually diminish to zero at the grapefruit's maximum height, after which the grapefruit will experience only the downward force of its weight.
(C) both an upward force and the downward force of its weight as it rises. The upward force will gradually diminish to zero at the grapefruit's maximum height, after which the grapefruit will experience only the downward force of its weight.
(D) only the downward force of its weight, both before and after it reaches its maximum height.
Answer: (D) only the downward force of its weight, both before and after it reaches its maximum height.
Why: Once it leaves your hand, the grapefruit coasts upward. There is nothing pushing it upward. The only force acting on the grapefruit its weight and perhaps some downward air resistance (drag).
Problem 7:
You are taking a shower in your dormitory when someone flushes a toilet nearby. The pressure in the cold water line drops and you find yourself showering in what feels like molten lava. This loss of cold-water pressure occurs when the flushing toilet lets more cold water flow through the pipes delivering it to the bathroom and the water's speed in those pipes increases. The cold water's faster motion in the delivery pipes reduces its pressure in the showerhead because faster moving water
(A) has less kinetic energy than slower moving water.
(B) losses more energy to viscous drag as it flows through the delivery pipes.
(C) has less pressure than slower moving water.
(D) has less gravitational potential energy than slower moving water.
Answer: (B) losses more energy to viscous drag as it flows through the delivery pipes.
Why: If there were no way to lose energy in the pipes, the cold water reaching the shower would always maintain a high pressure and form a strong spray. But as the speed in the cold water pipes increases, the water does lose energy and by the time it reaches the showerhead, it just doesn't have much energy left. The hot water then dominates the flow and you get scorched.
Problem 8:
You are practicing shooting free throws at the basketball court. When you throw the ball, it travels in an arc toward the hoop. Ignoring any forces that air exerts on the ball, the net force on the ball just after it leaves your hand is
(A) down and away from you.
(B) up and away from you.
(C) straight down.
(D) zero.
Answer: (C) straight down.
Why: Only gravity acts on the ball and gravity pulls the ball straight down. Even with air resistance included, the forces on the ball are never forward and thus never point away from you.
Problem 9:
You are swimming at the beach. You swim out past the breaking surf so that you can float among the waves. Each time a wave crest passes, you travel
(A) vertically—directly up and then directly down.
(B) horizontally toward shore for a distance that's proportional to the slope of the wave crest.
(C) in a circle—up, toward shore, down, and away from shore.
(D) horizontally toward shore for a distance equal to one wavelength of the passing wave.
Answer: (C) in a circle—up, toward shore, down, and away from shore.
Why: Each passing wave crest is built from local water. Since you are submerged in that water, you follow it as it temporarily forms the crest. The overall motion of surface water as the crest passes is a circular one and you travel along with it.
Problem 10:
Two steel balls, one of which weighs twice as much as the other, roll off of a horizontal table with the same speeds. In this situation,
(A) the lighter ball impacts the floor at about half the horizontal distance from the base of the table than does the heavier.
(B) the heavier ball impacts the floor at about half the horizontal distance from the base of the table than does the lighter.
(C) both balls impact the floor at approximately the same horizontal distance from the base of the table.
(D) the heavier ball hits considerably closer to the base of the table than the lighter, but not necessarily half the horizontal distance.
Answer: (C) both balls impact the floor at approximately the same horizontal distance from the base of the table.
Why: The balls continue to coast horizontally at the same speed while they are falling. Since they take the same amount of time to fall, they hit the ground at the same time and having traveled horizontally the same distance.
Problem 11:
A flashlight equipped with new batteries produces bright, yellow-white light. As the batteries in the flashlight wear out, the bulb will
(A) suddenly go out even if the filament doesn't break.
(B) glow dimly and produce reddish light.
(C) continue to glow brightly, but produce redder light.
(D) glow dimly, but continue to produce yellow-white light.
Answer: (B) glow dimly and produce reddish light.
Why: The filament will become colder and colder objects emit (1) less light and (2) redder light (longer wavelength; not as much blue).
Problem 12:
A pedestrian bridge crosses Emmet Street near Ruffner Hall (the Education School). This bridge is entirely supported by columns from below. A gap at each end of the bridge separates the bridge's surface from the sidewalks leading to the bridge. The width of each gap changes with time. This width is smallest
(A) on hot days.
(B) at noon.
(C) at midnight.
(D) on cold day.
Answer: (A) on hot days.
Why: On hot days, the bridge heats up and expands. The ground maintains a roughly constant temperature, so the spacing of the two sidewalks remains the same. That means that bridge almost fills the gaps on the hottest days.
Problem 13:
One warm spring day you and some friends go to the beach at a nearby lake. You are tossing a soft, almost fully inflated beach ball around when someone accidentally knocks it into the lake. Although it is a warm day, the water is still very cold so nobody wants to retrieve the ball. You notice that the ball seems to have deflated somewhat after sitting in the water for a while. The contact with the cool water has caused the temperature of the air inside the ball to drop, resulting in
(A) a decrease in the pressure inside the ball.
(B) an increase in the density of the air inside the ball.
(C) a decrease in the density of the air inside the ball.
(D) an increase in the pressure inside the ball.
Answer: (B) an increase in the density of the air inside the ball.
Why: The pressure in the ball is essentially atmospheric throughout. That's because the ball's skin isn't taut and can't sustain any pressure difference between inside and outside. When the ball is cooled, the air inside doesn't change pressure. However, with less thermal energy to assist them, the molecules must pack more tightly together to maintain atmospheric pressure. The density in the ball rises.
Problem 14:
The strings of a guitar vibrate at different frequencies. They have different thicknesses because
(A) a thinner string has more tension than a thicker string and vibrates more quickly.
(B) a thicker string has more tension than a thinner string and vibrates more quickly.
(C) a thicker, more massive string vibrates more slowly than a thinner string of the same length and tension.
(D) a thinner, less massive string vibrates more slowly than a thicker string of the same length and tension.
Answer: (C) a thicker, more massive string vibrates more slowly than a thinner string of the same length and tension.
Why: A string can have any tension you like (as long as you don't overtighten it and break it). But increasing the thickness and mass of the string will slow its motion by giving it more inertia.
Problem 15:
A child is playing on a swing. As long as he doesn't swing too high the time it takes him to complete one full oscillation will be independent of
(A) the weight of the child, but not the amplitude of the motion.
(B) both the amplitude of the motion and the weight of the child.
(C) both the length of the ropes supporting the swing and the amplitude of the motion.
(D) the amplitude of the motion, but not the weight of the child.
Answer: (B) both the amplitude of the motion and the weight of the child.
Why: As a harmonic oscillator, a swing's period is independent of its amplitude. But a pendulum is a special harmonic oscillator: since its restoring force is proportional to its mass and its inertia is proportional to its mass, mass cancels out and the period is independent of mass. A heavy child and a light child swing with the same periods.
Problem 16:
You're operating a propane gas grill from a fuel tank that's located below the grill. This tank is half filled with liquid propane. As the grill consumes gaseous propane, the tank's temperature drops below room temperature. This refrigerating effect occurs because
(A) heat naturally flows from a cooler object to a hotter object.
(B) heat is needed to initiate and sustain combustion in the grill.
(C) thermal energy is being used to convert liquid propane into gaseous propane in the tank.
(D) heat naturally rises from the tank to the grill above it.
Answer: (C) thermal energy is being used to convert liquid propane into gaseous propane in the tank.
Why: As the grill removes gaseous propane from the tank, it upsets the equilibrium between liquid and gas in that tank. The gas's density drops and there are then too few molecules returning to the liquid phase each second to keep the liquid from evaporating. As the liquid evaporates, thermal energy is used to separate its molecules and the temperatures of the liquid and the tank drop.
Problem 17:
Two boys are swinging a long jump rope while a girl jumps in the middle. Despite its overall rotating motion, the jump rope is actually vibrating up and down in its fundamental mode while the boys circle their hands gently to keep the rope vibrating. To speed up the jump rope's vibration so that the girl has to jump more often, the boys must circle their hands more quickly and
(A) reduce the tension in the rope.
(B) increase the tension in the rope.
(C) decrease the height (amplitude) of the arc formed by the swinging rope.
(D) increase the height (amplitude) of the arc formed by the swinging rope.
Answer: (B) increase the tension in the rope.
Why: The jump rope is a giant vibrating string. Like any string, its frequency of vibration is determined by its tension, its length, and its mass per unit of length. The tauter the rope is, the faster it vibrates and that is how the boys make it vibrate faster: they increase the tension.
Problem 18:
A popular holiday house ornament consists of a little metal fan with horizontal blades, that pivots above a couple of burning candles. As the candles burn, their heat makes the fan spin and little hammers that move with the fan ring several tiny bells. While the decoration runs on heat alone, it won't work when you remove the candles and put it inside a hot, dark oven. It's inability to use the oven's heat is explained by the fact that
(A) the candles produced burned gases while the oven contains only hot air.
(B) the candle's heat is ordered, while the heat in the oven is completely disordered.
(C) the candle flames emit light, while the oven is dark.
(D) both a hot region and a cold region are needed in
order to convert heat into work.
Answer: (D) both a hot region and a cold region are needed in order to convert heat into work.
Why: The ornament is a heat engine and as such, it converts a small amount of heat into work as heat flows from a hot object (the candle flames) to a cold object (the room air). More specifically, it is using convection to turn the fan blades. In an oven at a uniform temperature, there is no convection. And from a thermodynamic standpoint, with no cold object around, it's not possible to convert thermal energy into ordered energy.
Problem 19:
When you blow gently across the top of a particular soda bottle, it emits a tone. The column of air in the bottle is vibrating up and down in its fundamental mode. If you replace the air in the bottle with helium and then blow gently across the top of the bottle, it will emit
(A) a higher pitched tone.
(B) a tone at the same pitch as before.
(C) no sound at all.
(D) a lower pitched tone.
Answer: (A) a higher pitched tone.
Why: All of the forces involved in the gas vibrations will remain the same, but the mass of the moving gas column will decrease. As a result, that column will go through its motions faster and emit a higher-pitched tone.
Problem 20:
Two of your friends are bungee jumping from Beta Bridge. After the initial fall and rebound, each jumper bounces up and down as a harmonic oscillator—a mass on the end of spring. The period of the oscillator is the time between successive bounces. One friend weighs twice as much as the other. Since they use the same bungee cord for their jumps,
(A) the two friends will have the same period, regardless of how high they bounce.
(B) the heavier friend will have a longer period than the lighter friend.
(C) the two friends will have the same period, as long as the height of their bounces are the same.
(D) the heavier friend will have a shorter period than the lighter friend.
Answer: (B) the heavier friend will have a longer period than the lighter friend.
Why: The period of a harmonic oscillator increases as the mass of the oscillating object increases. With more inertia, the object simply goes through its motions more slowly. In this case, the heavier person has more mass and bounces up and down more slowly.
Problem 21:
Suppose that a shoelace passes without friction through 5 holes on each side of your shoe. If the tension in that shoelace is 10 N, how much total force is the shoelace exerting on the right side of your shoe?
(A) 100 N
(B) 10 N
(C) 50 N
(D) 25 N
Answer: (A) 100 N
Why: Each segment of lace pulls on the right side of your shoe with 10 N of force. Since there are 10 segments of lace reaching over toward the right side of your shoe (two for each hole), that is 10 times 10 N or a total of 100 N.
Problem 22:
You are surfing on a large wave. You are half way up the front edge of the wave and maintaining a steady height. You and the wave are moving steadily toward shore at a constant speed. The net force on you is
(A) downhill—toward the trough in front of you.
(B) zero.
(C) uphill—toward the crest behind you.
(D) horizontal—toward the shore.
Answer: (B) zero.
Why: You are moving at a constant speed and moving in a fixed direction, so you are at constant velocity. Since you are thus not accelerating, you have zero net force acting on you.
Problem 23:
You are mountain biking through the woods and come to a deep gully. The bottom of the gully is a smooth bowl shape without any obstacles. Neglecting friction and air resistance, you are able to coast down one side and back up the other without pedaling. When you are at the bottom of the gully, you feel
(A) heavier than normal since you are accelerating upward.
(B) lighter than normal since you are accelerating upward.
(C) no heavier or lighter than normal since you are not accelerating at the bottom.
(D) heavier than normal since you are accelerating downward.
Answer: (A) heavier than normal since you are accelerating upward.
Why: Your velocity is changing from slightly downward to slight upward as you go through the bottom of the bowl. That requires some amount of upward acceleration. Since fictitious forces always appear in the direction opposite acceleration, you feel pulled downward and thus extra heavy.
Problem 24:
You are at the park throwing a stick for your dog to fetch. When you throw a smooth straight stick by one end, it tends to tumble end over end as it flies through the air. However, when you throw a stick that still has some leaves attached to one end, it flies through the air without tumbling and travels in a smooth arc with the stick leading and the leaves trailing. This automatic alignment occurs because
(A) the center of mass of the stick is no longer near its geometric center.
(B) the center of pressure is at the same point as the center of mass in the stick, stabilizing its flight.
(C) the viscosity of the air near the leaves is highest, producing torques which oppose any rotation the stick had after leaving your hand.
(D) the drag forces acting on the leafy stick are not uniform around the stick's center of mass.
Answer: (D) the drag forces acting on the leafy stick are not uniform around the stick's center of mass.
Why: Different parts of the leafy stick catch the wind different and experience different drag forces. There is a general imbalance to these drag forces that tends to twist the leafy end toward the back—away from the direction of the stick's motion.
Problem 25:
On a windy day you notice that a tall light pole is bent away from its equilibrium position. If the wind speed were to increase the pole would
(A) remain bent by the same amount since aerodynamic drag forces, like sliding friction, are independent of the relative speed of the objects.
(B) straighten out because the airflow would become laminar and reduce the drag force.
(C) remain bent by the same amount because the larger drag force will be canceled by a larger restoring force.
(D) bend further from its equilibrium position because the aerodynamic drag forces that are bending the pole will increase with the speed of the air moving around the pole.
Answer: (D) bend further from its equilibrium position because the aerodynamic drag forces that are bending the pole will increase with the speed of the air moving
Why: Unlike sliding friction, the force of air resistance (drag) depends on the relative velocity of the air and object. As the air speed past the object increases, the drag force increases and, in this case, the pole bends farther before experiencing enough restoring force to keep it motionless.
Problem 26:
You construct a water-powered go-cart by attaching a large tank of water to a cart. The water can be forced out a nozzle by connecting a tank of high-pressure air to the water tank. In preparation for your first ride, you position this contraption facing away from a brick wall. You climb on and start spraying the water against the wall. To your delight you accelerate away from the wall and begin to move. When you have moved far enough away from the wall that the stream of water can no longer hit it, you
(A) continue to accelerate as the water sprays out the back of the cart.
(B) continue to move, but stop accelerating since the water can no longer push against the wall.
(C) feel lighter than normal since the water hitting the ground will produce an upward force.
(D) continue to accelerate, but less since it takes more force to accelerate a moving object than a stationary one.
Answer: (A) continue to accelerate as the water sprays out the back of the cart.
Why: The cart accelerates because it pushes the water backward, not because the water hits the wall. Whether or not that water hits anything makes no difference to the cart's motion; it accelerates as long as it pushes the water backward and the water pushes it forward in response.
Problem 27:
You are enjoying an elegant candlelit dinner for two but the waiter has been a little careless with the ice. Your glass of ice water contains 25% water and 75% ice, while your friend's glass contains 75% water and 25% ice. The temperature of your glass of ice water
(A) is higher than that of your friend.
(B) is the same as that of your friend.
(C) is lower than that of your friend.
(D) depends on when it was filled. The glass that was filled first will now be warmer than the glass that was filled second.
Answer: (B) is the same as that of your friend.
Why: Both glasses contain water and ice in equilibrium. That equilibrium can only occur when both materials are at 0 ºC—the melting temperature of ice or the freezing temperature of water.
Problem 28:
You can put your hand in a 400° F oven briefly without getting burned. However, even a second or two of contact with 212° F steam can be quite painful. The steam delivers heat much faster to your hand because steam
(A) undergoes much more rapid convection than air.
(B) condenses into water and deposits its heat of vaporization into your hand.
(C) is a gas with a much higher thermal conductivity than air.
(D) is actually a liquid with a high thermal conductivity. It is composed of countless tiny droplets suspended in the air.
Answer: (B) condenses into water and deposits its heat of vaporization into your hand.
Why: As steam touches your skin, it finds itself far from equilibrium. Very few water molecules leave the surface of your skin because of its low temperature and many water molecules hit the surface of your skin because of its high density in the vapor. There is a large net flow of water molecules from the gas phase to the liquid phase. As these molecules condense to liquid on your skin, they release chemical potential energy: the heat of vaporization. The temperature of your skin rises quickly and you feel pain.
Problem 29:
When you get new tires installed on your car the wheels are carefully balanced by placing small weights around the outside of the rim. If one of these weights falls off, your car may vibrate violently when you drive at certain speeds. This vibration occurs for speeds at which
(A) the vibration frequency of the off-balance wheel matches the frequency at which the car likes to vibrate allowing a resonant energy transfer between the two.
(B) the rotational kinetic energy of the wheel exactly matches the translational kinetic energy of the car, allowing a resonant energy transfer between the two.
(C) the vibrating wheel acts like a harmonic oscillator.
(D) the amplitude of the vibrating wheel becomes large enough to overcome the inertia of the massive car.
Answer: (A) the vibration frequency of the off-balance wheel matches the frequency at which the car likes to vibrate allowing a resonant energy transfer between the two.
Why: Your car can vibrate in various ways if something gets it started. In this case, the off-balance wheel can drive the car's vibration by resonant energy transfer. Energy flows from the wobbling wheel to the vibrating car.
Problem 30:
Two-part epoxy is a versatile adhesive that can be used to glue things together. Unlike most glues, it consists of two liquids that must be mixed together. When the liquids are combined, the mixture becomes warm indicating that a chemical reaction is taking place in which
(A) the bonds holding together the molecules in the reacted mixture are stronger that the bonds holding together the molecules of the unmixed liquids.
(B) the bonds holding together the molecules in the reacted mixture are weaker that the bonds holding together the molecules of the unmixed liquids.
(C) heat is flowing against its naturally preferred direction, from the room into the epoxy.
(D) chemical potential energy is increasing.
Answer: (A) the bonds holding together the molecules in the reacted mixture are stronger that the bonds holding together the molecules of the unmixed liquids.
Why: The hardening epoxy is releasing chemical potential energy as its molecules rearrange into ones with less total chemical potential energy. The newly created molecules are more tightly bound together, meaning that it would take more work to disassemble them into their constituent atoms. The chemical potential energy that was released in the reaction becomes thermal energy and warms the hardening glue.
Problem 31:
Mike and Johnny are in trouble for scuffling in the school cafeteria. While Mike admits that he pushed Johnny, who immediately fell over backward, Mike claims that Johnny pushed back and is thus just as guilty. From the perspective of physics,
(A) Johnny pushed back on Mike, but with less force than Mike exerted on him.
(B) Johnny did push back on Mike, with exactly the same amount of force.
(C) Johnny didn't push back on Mike.
(D) Johnny pushed back on Mike, but with more force than Mike exerted on him.
Answer: (B) Johnny did push back on Mike, with exactly the same amount of force.
Why: For every force that Mike exerts on Johnny, there is an equal but oppositely directed force that Johnny exerts on Mike. That's simply Newton's third law and the way the universe works.
Problem 32:
The brake system in most cars makes use of a hydraulic system. This system consists of a fluid filled tube connected at each end to a piston. Assume that the piston attached to the brake pedal has a cross sectional area of one half a square inch and the piston attached to the brake pad has a cross section area of two square inches. When you apply a force of 10 pounds to the piston attached to the brake pedal, the force at the brake pad will be,
(A) 5 pounds.
(B) 10 pounds.
(C) 20 pounds.
(D) 40 pounds.
Answer: (D) 40 pounds.
Why: The pressure in the fluid filled tube will be essentially uniform, so if it exerts 10 pounds on a half a square inch piston, it will exert 40 pounds on a two square inch piston. The second piston has 4 times the surface area and therefore experience 4 times the force from the fluid.
Problem 33:
You are filling a large lightweight dry cleaning bag with helium. At first, the plastic bag doesn't float. But as you keep adding helium to the bag, it eventually begins floating because
(A) at the same pressure and temperature, the upward buoyant force on a helium-filled bag is larger than the buoyant force on an air-filled bag with the same volume.
(B) the helium-filled bag's weight decreases as you put more lightweight helium particles inside it.
(C) the average density of the helium-filled bag decreases even though the buoyant force on the bag remains constant as it fills.
(D) as the volume of displaced air increases, the buoyant force increases until it is greater than the weight of the helium-filled bag.
Answer: (D) as the volume of displaced air increases, the buoyant force increases until it is greater than the weight of the helium-filled bag.
Why: Adding helium to the bag increases its weight slowly but surely, but the bag's volume increases relatively quickly. Eventually the bag displaces so much air that the buoyant force on it (the weight of the displaced air) exceeds its weight and up it goes.
Problem 34:
Moments before it's ignited by the sparkplug, the mixture of fuel and air inside an automobile cylinder is compressed to very high density. During the compression process, the mixture's
(A) temperature stays the same but its pressure rises.
(B) temperature rises dramatically and so does its pressure.
(C) temperature drops dramatically but its pressure rises.
(D) temperature rises dramatically but its pressure drops.
Answer: (B) temperature rises dramatically and so does its pressure.
Why: When you compress a gas, you do work on it and also increase its density. Doing work on it increases its thermal energy and thus its temperature. Increasing its density and temperature both cause its pressure increase.
Problem 35:
A thermoelectric cooler is a type of heat pump that uses electric power to move heat against its natural direction of flow. In such a heat pump electrical power must be converted to thermal energy because,
(A) the entropy of the system must be conserved.
(B) Newton's laws of motion prevent heat from moving from a cool region to a warm region.
(C) the flow of heat from a cool region to a warm region alone would decrease the entropy (disorder) of the system.
(D) the flow of heat from a cool region to a warm region alone would violate the conservation of energy.
Answer: (C) the flow of heat from a cool region to a warm region alone would decrease the entropy (disorder) of the system.
Why: If a heat pump could move heat from colder to hotter without any other effects, it would be decreasing the overall entropy of the universe. That action would be incredibly unlikely and would therefore violate the 2nd law of thermodynamics. While not strictly forbidden by the laws of motion, this sort of action never occurs in practice. That's why the heat pump must create extra entropy by converting ordered energy into disordered energy as part of its operation.
Problem 36:
It's a windy day and there are waves on the surface of the open ocean. The wave crests are 40 feet apart and 5 feet above the troughs as they pass a school of fish. The waves push on fish and making them accelerate. The fish don't like this jostling, so to avoid it almost completely the fish should swim
(A) as fast as possible.
(B) at least 5 feet below the surface of the water.
(C) as close to the surface of the water as possible.
(D) at least 40 feet below the surface of the water.
Answer: (D) at least 40 feet below the surface of the water.
Why: Surface waves influence the motion of water to about a wavelength below the surface. In this case, the wavelength is about 40 feet, so the influence is limited to about 40 feet of depth. (Note: I have recently been informed that the influence is even shallower, that it extends to only about half the wavelength. Thus a more appropriate answer would be 20 feet of depth. The second edition of the book will reflect this change. -- LB)
Problem 37:
The glass envelope of an ordinary incandescent light bulb is filled with a low-pressure mixture of nitrogen and argon gases. If there were no gas at all inside a bulb's envelope, this special bulb would be
(A) more energy efficient but would have a shorter operating life than an ordinary bulb.
(B) less energy efficient and would have a shorter operating life than an ordinary bulb.
(C) less energy efficient but would have a longer operating life than an ordinary bulb.
(D) more energy efficient and would have a longer operating life than an ordinary bulb.
Answer: (A) more energy efficient but would have a shorter operating life than an ordinary bulb.
Why: The gas slows the sublimation of tungsten atoms and keep the filament intact longer. However it wastes heat via conduction to the bulb's walls.
Problem 38:
You are frosting some holiday cookies using a tube of frosting with a long narrow tip attached. You must squeeze the tube very hard to get the frosting to come out of the narrow opening because,
(A) viscous drag between the walls of the tip and the frosting waste considerable energy as heat.
(B) viscous drag between the walls of the tip and the frosting causes the frosting to swirl around chaotically.
(C) Newton's third law requires most of the energy in the frosting to be used to push back on you rather than moving it through the tip.
(D) the high density of the frosting impedes its flow through the small opening.
Answer: (A) viscous drag between the walls of the tip and the frosting waste considerable energy as heat.
Why: The viscous frosting experiences nearly perfect laminar flow. However, its high viscosity means that the walls of the tip influence not only the frosting that touches the walls, but also the frosting far away as well. You have to squeeze hard to overcome this viscous drag problem and move the frosting through the tip at a reasonable rate.
Problem 39:
To improve a xylophone's appearance, the orchestra director decides to have it painted. Each xylophone bar is actually a harmonic oscillator that vibrates when struck by a wooden mallet. When bending in its fundamental mode, the bar's middle and ends move in opposite directions. Painting the bar adds mass to the bar without affecting its stiffness, so the paint will
(A) not affect the pitch or sound volume of the bar.
(B) raise the bar's pitch.
(C) lower the bar's pitch.
(D) not affect the pitch of the bar, but will reduce its sound volume.
Answer: (C) lower the bar's pitch.
Why: The bar is a harmonic oscillator. The stiffness of the bar provides the restoring force and the mass of the bar provides the inertia. If you simply increase the inertia, you slow all the accelerations and length the period of oscillation. The bar's pitch (frequency of vibration) will go down.
Problem 40:
A new bowling alley has just opened in which the lanes are not horizontal, but slope upward slightly from where you release the ball to where the pins are standing. The lanes in this new bowling alley are highly polished, so there is no friction between the ball and the floor. Moreover, the ball never moves fast enough for air resistance to effect its motion. Once you release the ball and allow it to roll on its own up one of these lanes, the ball
(A) moves uphill because of the uphill acceleration on the ball.
(B) moves with a constant velocity because of Newton's first law; an object in motion tends to remain in motion.
(C) slows down because of the downhill acceleration on the ball.
(D) decelerates because of the uphill velocity of the ball.
Answer: (C) slows down because of the downhill acceleration on the ball.
Why: The ball's velocity changes as it accelerates. On a lane that slopes uphill, the acceleration (which is always downhill) is back toward you. The ball accelerates toward you as it rolls and therefore losses forward speed.
Problem 41:
A fan can be used to circulate air around a room. The pressure at the inlet side of the fan is
(A) less than the pressure at the outlet side and less than atmospheric pressure..
(B) less than the pressure at the outlet side but more than atmospheric pressure.
(C) equal to the pressure at the outlet side.
(D) more than the pressure at the outlet side.
Answer: (A) less than the pressure at the outlet side and less than atmospheric pressure..
Why: The fan does work on the air passing through it, in this case raising its pressure potential energy. (It may also speed the air up, too, by creating nozzle effects.) Since room air spontaneously flows into the fan to be pushed forward by it, the pressure at the inlet of the fan must be below atmospheric pressure. That way, room air accelerates toward the fan's inlet and is then pushed through it and across the room.
Problem 42:
You place three nonflammable objects in a fire. They are identical in shape and size, but one object is black, the second is white, and the third is shiny silver. After a few minutes, all three objects are at the same temperature: 1800° C. They remain solid and are now glowing with thermal radiation. Which one is glowing most brightly?
(A) The first object (black).
(B) They are all glowing with equal brightness.
(C) The third object (silver).
(D) The second object (white).
Answer: (A) The first object (black).
Why: A black object is good at interacting with visible light. It absorbs visible light beautifully, which is why it appears black. It also emits visible light beautifully when hot, which is why it glows brightly.
Problem 43:
After clearing the bar in the high jump, you land softly on a giant mattress. Landing on the mattress is much more comfortable than landing on a sand heap of equal size because
(A) you transfer less momentum to the mattress in coming to a stop than you would have transferred to the sand heap in coming to a stop.
(B) the force that the mattress exerts on you to stop your descent is much less than the force that the sand heap would have exerted on you.
(C) you transfer more momentum to the mattress in coming to a stop than you would have transferred to the sand heap in coming to a stop.
(D) your velocity is less as you land on the mattress than it would have been if you'd landed on the sand heap.
Answer: (B) the force that the mattress exerts on you to stop your descent is much less than the force that the sand heap would have exerted on you.
Why: No matter what you hit, you will transfer all of your downward momentum to it eventually. But if you hit a soft surface like that of the mattress, this momentum transfer will be slow and involve small forces. The impulse (force times time) will always be the same, but you might as well minimize force by maximizing time.
Problem 44:
You're the first person to visit Mars and you've just met a group of Martian school children. They are playing street hockey with parts from the Mars Polar Lander. After years of watching Star Trek reruns, they are fluent in English. One of them asks you how your weight and mass have changed since you left earth. You take a moment to measure both and reply correctly that
(A) your mass is still essentially unchanged but your weight is less than on earth.
(B) your weight is still essentially unchanged but your mass is less than on earth.
(C) neither your weight nor your mass have changed much.
(D) your weight and mass have both changed significantly.
Answer: (A) your mass is still essentially unchanged but your weight is less than on earth.
Why: Your mass doesn't depend on gravity or where you are located. You are just as hard to shake on Mars as you are on Earth. But your weight depends on the local gravity. Since Mars is smaller than Earth and it has weaker gravity, you weigh less there.
Problem 45:
You are in the kitchen with three mixing bowls in front of you. One bowl is metal, the second is glass, and the third is plastic. All three are at exactly the same temperature: the 68° F (20° C) temperature of the room. If you touch the three bowls together,
(A) heat will flow from the glass bowl to both the plastic bowl and the metal bowl.
(B) heat will flow from the plastic bowl to the glass bowl, and from the glass bowl to the metal bowl.
(C) no heat will flow between the bowls.
(D) heat will flow from the metal bowl to the glass bowl, and from the glass bowl to the plastic bowl.
Answer: (C) no heat will flow between the bowls.
Why: The fact that all three bowls have the same temperature means that they are in thermal equilibrium and that no heat will flow between them when they touch.
Problem 46:
On a visit to the pet store you notice that some fish are able to hover motionless near the center of the tank, even without moving their fins or tail. This indicates that
(A) the fish is dead.
(B) the density of the fish is less than the density of the water.
(C) the density of the fish is greater than the density of the water.
(D) the density of the fish is equal to the density of the water.
Answer: (D) the density of the fish is equal to the density of the water.
Why: The fish are maintaining neutral buoyancy: they neither sink nor float. This achievement is difficult and requires that the fish match their weights and buoyancies perfectly. They have the same density as water. As a result, the net force on them is zero. The buoyant force upward balances the weight downward.
Problem 47:
You and a friend are wondering how your weights compare. You don't have a scale, but you do have a piece of rope. To compare your weights you throw the rope over the branch of a tree and each hang from one end of the rope. To your surprise, when you both pick your feet up the rope remains motionless indicating that your weights are exactly the same (neglecting any friction between the rope and the branch). From this observation you know that the tension in the rope is
(A) equal to four times your weight.
(B) equal to half your weight.
(C) equal to your weight plus your friend's weight (or twice your weight).
(D) equal to your weight.
Answer: (D) equal to your weight.
Why: The net force on you as you hang motionless is clearly zero. Your weigh must therefore exactly balance the tension force upward. Tension must thus equal your weight.
Problem 48:
You place a bottle of water over a burner and bring it to a rapid boil. You then cover the bottle with a stopper and remove it from the heat. As soon as the water stops boiling you push the stopper snugly into the opening to seal the bottle, which now contains no air, only liquid water and steam. If you let the sealed bottle cool to room temperature,
(A) the pressure will remain the same as when you sealed it since the rate at which water evaporates balances the rate at which it condenses.
(B) a partial vacuum will form since the vapor pressure of water at room temperature is much less than atmospheric pressure.
(C) the top will blow off of the bottle as water continues to evaporate, causing the pressure inside to rise.
(D) the water will begin to boil again since a vacuum will form and water in a vacuum boils.
Answer: (B) a partial vacuum will form since the vapor pressure of water at room temperature is much less than atmospheric pressure.
Why: As the water cools, the equilibrium between gas and liquid will change. At room temperature, water molecules will leave the liquid more slowly and there can't be as many water molecules in the gas phase. The density of the vapor will decrease.
Problem 49:
If you blow across the top of a half full bottle of soda you can produce a clear tone. If you take a drink of soda to reduce the amount of liquid in the bottle and try this again the pitch (frequency) of the sound produced will
(A) increase.
(B) stay the same.
(C) exactly double.
(D) decrease.
Answer: (D) decrease.
Why: The column of air in the bottle will get taller. Its mass will increase and the pressure gradients will get weaker. Overall, the accelerations of the air column will slow and the frequency of oscillation (pitch) will drop.
Problem 50:
It's a warm summer day and you're having lunch outdoors on a patio. A window air conditioning unit hums quietly nearby as it cools an office inside the building. Your friend notices that the unit's outdoor part is emitting a considerable amount of heat and comments on how strange that is. You explain correctly that the air conditioner's outside part is emitting the heat
(A) it produces from the electricity it consumes.
(B) that is left over when it converts thermal energy from the inside air into electricity.
(C) it removes from the inside air and the heat it produces from the electricity it consumes.
(D) it produces during its defrost cycle; when it warms up its evaporator to remove ice that forms because of the humidity.
Answer: (C) it removes from the inside air and the heat it produces from the electricity it consumes.
Why: The air conditioner is a heat pump, removing heat from room air and dumping it into the outdoor air. To make this transfer, the air conditioner also consumes electric energy and converts that energy into thermal energy. This thermal energy is also released into the outdoor air.
PART II: SHORT ANSWER QUESTIONS
Please give a brief answer in the space provided. Words written outside of the allotted space will not be read during grading. Part II is worth 33% of the grade on this examination.
Problem 1:
You are sitting by the pool on a hot summer day, sipping a glass of ice water.
(A) Why is the ice floating near the surface of the water?
Answer: The density of ice is less than the density of water, so the upward buoyant force on it when fully submerged exceeds its weight.
Why: The ice is in equilibrium. The upward buoyant force exerted on it by the water and air it is displacing exactly balances its downward weight.
(B) You notice that droplets of water are forming on the outside surface of the glass. What does this tell you about the relative humidity (the measure of the condensation rate as a percentage of the evaporation rate) at the surface of the glass?
Answer: The relative humidity near the surface of the glass exceeds 100%.
Why: The water is condensing on the glass, meaning that the rate at which molecules land on the surface exceeds the rate at which they leave. That means that the relative humidity at the surface exceeds 100% because the balance is perfect at 100%.
(C) When you put the glass down on the hot surface of a black table the ice-water mixture remains at a uniform temperature of 0° C even thought heat is flowing from the hot table to the cold ice water. What is happening to the thermal energy that is being added to the ice water?
Answer: It is being used to convert ice into water.
Why: There is a large latent heat of fusion that must be added to the ice in order to melt it, even without changing the temperature of the system.
Problem 2:
Some distant relatives have left you something in their will: a company that manufactures bathroom scales. Sadly, the company has fallen on hard times because their scales aren't popular anymore. You begin to examine the current model and soon realize that whoever designed it had never taken a course in basic physics. You make a few changes and soon have business booming again. Let's take a look at why the scale was flawed prior to your changes.
(A) The scale uses a spring to measure weight. However, to save money, the scale uses a cheap spring that gradually loses its stiffness. As the years pass, its restoring force gets weaker and weaker. Will this flaw affect the scale's reading and, if so, will the scale read heavier or lighter as the years pass?
Answer: The scale will read heavy.
Why: As the spring weakens, it will distort more and more when supporting the same weight. Since the value the scale reports is proportional to its spring's distortion, the scale will report the excessive distortion as a larger weight than is actually present.
(B) Its soft rubber base gives the scale a pleasant "bouncy" feeling when you step on it. But the bouncing base also makes the scale's needle swing back and forth so that it's hard to read your weight. Why does the up and down motion of the scale cause the needle to swing?
Answer: The scale accelerates as it bounces and as it tries to make you accelerate with it, it pushes on you with forces that alternately exceed and fall below your actually weight. The scale reports its fluctuating support force by jiggling its needle back and forth.
Why: The scale's needle simply reports the force it is exerting on you. If the scale is accelerating up and down, then you are too and the force it's exert on you, along with its needle's reading, fluctuates.
(C) The top surface of each scale is made of shiny copper metal. The copper looks great but feels unpleasantly cold when you step on it in bare feet. You redesign the scale to use a copper-colored plastic top and the scale becomes much more comfortable to step on. Why does the plastic top feel warmer than the metal top, even when they are at the same temperature?
Answer: The metal conducts heat away from your feet more quickly than plastic.
Why: Your feet send heat into the scale's top surface because of the difference in temperature between your warm feet and the cold surface. The plastic surface heats quickly to body temperature because it doesn't conduct heat well and the thermal energy you give the surface stays there. But metal conducts heat much more effectively and so your thermal energy is quickly distributed through the scale's top. You have to heat the whole metal piece and that requires more thermal energy. Your feet experience a heavy demand for heat and they feel cold.
Problem 3:
It's your first day on the staff of the U.S. Patents Office and you're excited about having some new ideas come across your desk.
(A) An inventor comes to you with a small box that's supposed to make batteries obsolete. The inventor claims that the box can produce electricity forever without having to be recharged. You can be sure that this claim is nonsense because
Answer: it fails to conserve energy (or violates the 1st law of thermodynamics). (Note: alternative interpretation: it turns disordered energy into ordered energy (or violates the 2nd law of thermodynamics).
Why: Energy in any form can't flow endlessly out of an object without being replaced. That's essentially the 1st law. However, the question never stated whether heat is allowed to flow into the box, so then the issue is that heat cannot be turned into work continuously. That's essentially the 2nd law.
(B) Another inventor comes in with a small motor-like device that is powered by a burning candle. The inventor claims that this device takes the heat from the candle and turns it entirely into work. The device thus doesn't warm the room at all. Your knowledge of the laws of thermodynamics assures you that this claim, too, is nonsense because
Answer: it would then be turning thermal energy entirely into work, thereby decreasing the total entropy of the system.
Why: Turning thermal energy into work can't be done directly without decreasing the total entropy and violating the 2nd law of thermodynamics. Instead, heat must flow from a hotter object to a colder object, in which case a fraction of that heat can be diverted and converted into ordered energy. That's not the case here.
(C) As though the entire population of loser inventors was released on you in one day, another clown comes in claiming to have a heat pump that can transfer heat out of a box of corn flakes for as long as you like. The cereal just gets colder and colder. Once again, you know that this is impossible because
Answer: There is a minimum temperature: the cereal will approach (but never reach) absolute zero.
Why: Eventually the cereal will have given up all (or nearly all) its thermal energy and reach (or approach) absolute zero.
Problem 4:
You and a friend are playing pool (billiards) at a bar one evening. In this game, one ball (the cue ball) is used to knock the other balls into pockets around the edges of a table. For this problem assume that there is no friction between the balls and the table.
(A) It is your shot and you hit the cue ball toward one of the other balls. If the two balls have the same mass and the cue ball hits the other ball squarely, the cue ball will stop and the other ball will begin to move. What will the velocity (magnitude and direction) of the other ball be after the collision? (Answer by comparing to the original velocity of the cue ball.)
Answer: The second ball's final velocity will be exactly equal to the cue ball's initial velocity, both in magnitude and direction.
Why: Since momentum is conserved, the cue ball must give all of its initial momentum to the second ball. Since the two balls have identical mass and the momentum is equal to mass times velocity, the second ball's final velocity must equal the cue ball's initial velocity in order to conserve momentum properly.
(B) If the cue ball had half the mass of the other ball, would the cue ball remain stopped after the collision?
Answer: No.
Why: The cue ball would rebound from the other ball, much as it would if it rebounded from a wall.
(C) If the cue ball had half the mass of the other ball, which ball would experience a greater force during the collision?
Answer: They would experience equal forces (but in opposite directions).
Why: The two balls would repel one another with equal but opposite forces, in accordance with Newton's third law.
Problem 5:
You have recently arrived on the galactic megacruiser "Hollywood." This enormous spaceship was designed in part by unemployed screenwriters, so it has a few quirks that naturally occur when you ignore the laws of physics and build things according to what makes movie stars look their best.
(A) The ship has a huge flight deck from which small sightseeing spacecraft can depart. You watch from the flight office as tourists walk across the flight deck and board one of the little ships. The flight attendant shuts the passengers in and the flight controller immediately opens a huge door between the flight deck and empty space beyond. Suddenly there is a horrendous roar as air rushes out of the flight deck. Why does the air begin flowing toward the flight deck doors?
Answer: Air accelerates toward lower pressure. Since the pressure inside the deck is greater than in empty space, the air picks up speed toward the doors and sprays out into space.
Why: The air is initially motionless (more or less), so when it accelerates toward the lower pressure in space, it quickly begins heading in that direction.
(B) Think of the air in the flight deck as being in two halves: one nearest to the doors and one farthest from the doors. Use the definition of work to show that the half farthest from the doors does work on the half nearest to the doors.
Answer: The air farthest from the doors pushes the air nearest the doors toward those doors and the air nearest the doors moves toward those doors. Since work is force times distance in the direction of that force, the air farthest from the doors is doing work on the air nearest the doors.
Why: Pushing the air out the doors takes work and it comes from air farther from those doors. In the process of expelling the nearest air, the farther air expands, does work, and experiences a drop in both thermal energy and temperature.
(C) Even though it's turned off, the small ship is unceremoniously tossed into space moments after the doors open. What type of force causes the ship's spaceward acceleration?
Answer: Drag force (more specifically pressure drag).
Why: The ship is caught in a violent wind and air resistance (pressure drag) acts to keep it moving at the same velocity as the wind. As the air leaves the deck, so does the ship.
Problem 6:
You are part of a team designing an energy-efficient escalator system for a new department store. The store has two floors and patrons will ride between the floors on the escalator. Your team plans to use a single belt of stairs that will travel from the ground floor up to the second floor and then return to the ground floor in a perfectly symmetrical arrangement. The belt will then travel underneath the first floor and reemerge at its starting point. A single motor will turn the belt and convey all of the people up and down between floors.
(A) The belt moves at a very steady pace so that a person riding it upward toward the second floor travels at constant velocity. What is the amount and direction of the net force on that person?
Answer: Zero net force (no direction necessary).
Why: Since the person is moving at constant velocity, the net force on that person is zero. It has no direction.
(B) As that person rides upward toward the second floor, is there any (positive) work being done and, if so, is it being done by the person or by the belt of stairs?
Answer: Yes, the belt of stairs is doing (positive) work on the person.
Why: The stairs are pushing upward on the person (to support the person's weight) and the person is moving (at least partly) upward, so the stairs are doing work on the person.
(C) The total weight of patrons on the escalators is 10,000 newtons (about 2,200 pounds). Half the people (weight 5,000 newtons) are riding the upward escalator and half (weight 5,000 newtons) are riding the downward escalator. The belt advances 1 meter each second. Neglecting friction and air resistance, how much power must the motor provide to the belt?
Answer: Zero power.
Why: While the belt must do work on the people rising upward, the people descending do work on the belt. The amounts of work are equal, so overall, the descending people do just enough work on the belt to enable it to raise the rising people. The motor doesn't have to do any work at all (neglecting friction and air resistance, of course).
Problem 7:
You are pushing a child on a playground swing. She swings back and forth, completing one full cycle of motion every 5 seconds.
(A) What form(s) of energy is/are involved in the child's swinging motion?
Answer: Kinetic and gravitational potential.
Why: When the child is moving, she has kinetic energy. When she is higher than the minimum height, she has gravitational potential energy.
(B) You want to reduce the amplitude of her swing. When during each cycle should you push her gently forward in order to reduce the size of her swing?
Answer: Push her forward as she is coming toward you.
Why: If you push her forward as she moves backward (toward you), you do negative work on her and extract energy from her.
(C) How would you have to change the playground swing or the child in order to increase her period from 5 seconds to more than 5 seconds?
Answer: Lengthen the chains.
Why: The period of the swing is determined by the strength of gravity (which you can't change) and the length of the chains supporting the seat (which you can change). The longer the chains, the longer the period.
Problem 8:
You are competing in a bicycle race, but have some trouble along the way.
(A) One of your tires develops a slow leak. As the air slowly leaks out of the tire, why does the wheel become harder to keep moving at the same speed?
Answer: The tire's skin bends more and wastes more energy as thermal energy (due to internal friction in the rubber).
Why: You don't want to bend the tire much because that wastes energy. The rubber experiences internal friction that converts work into thermal energy. By keeping the tire properly inflated, you make it hard to dent and it wastes little energy bending and warming the rubber.
(B) Finally you notice what is happening and stop to fix the leak. What happens to the temperature of the air inside the tire as you quickly pump it up?
Answer: The temperature rises.
Why: Compressing air involves doing work on that air (pushing it inward as it moves inward) and the air's thermal energy and temperature rise as a result.
(C) When you get back on your bike you realize that you have left it in a gear where the chain is on a small crank sprocket (the gear attached to the pedals) and a large freewheel sprocket (the gear attached to the rear wheel). Does this make it easy or difficult to start pedaling?
Answer: Easy to start pedaling.
Why: The bicycle moves only a short distance with each pedal in this configuration, so little work is done on the bicycle with each turn of the crank. That means that the force involved in turning the crank will be small and the crank is easy to pedal.