Physics 105 - How Things Work - Fall, 2004
Final Examination - Solutions
Given Monday, December 20, 2004 from 9:00am to 12:00noon
Class Average = 66.0%, Class Median = 67.5%, Stdev = 12.4%
Problem 1:
During the power stroke in an internal combustion engine, the hot burned gases inside the cylinder push the piston out of the cylinder. As the piston moves out of the cylinder,
Answer: (D) both the pressure and the temperature of the hot burned gases decrease. [73.1% picked]
Why: The gas is doing work on the piston, pushing it outward as it moves outward. This work leaves the gas, which becomes cooler. Because the gas becomes cooler and also occupies more volume inside the cylinder, its pressure drops.
Problem 2:
An astronaut hovering in the middle of the space shuttle's cabin as it orbits the earth is
Answer: (B) accelerating directly toward the center of the earth. [73.6% picked]
Why: The astronaut is in free fall, accelerating toward the center of the earth at the full acceleration due to gravity.
Problem 3:
Someone has dropped a beach ball out of an airplane and it is descending toward the earth. The air pressure just beneath it as it descends is
Answer: (D) greater than atmospheric pressure. [71.1% picked]
Why: From the perspective of the descending ball, the air is flowing upward toward it and is spreading outward on its bottom surface. That bending of the airstreams away from the bottom of the ball is caused by an increased pressure near the bottom of the ball.
Problem 4:
When you pour water, it easily becomes turbulent. In contrast, when you pour honey it rarely becomes turbulent. Honey resists turbulence because
Answer: (A) its high viscosity (thickness) allows viscous forces to keep the flow orderly. [99.5% picked]
Why: Viscosity is the great ordering influence on fluids. The honey's increased viscosity allows it to stay orderly even as it flows around moderately large obstacles.
Problem 5:
You're at the county fair, watching people try to win prizes by tossing small hard-plastic rings onto the necks of glass soda bottles. The rings keep bouncing off the bottles and no one ever wins. What makes the game so hard to win is that the rings are unable to get rid of
Answer: (B) their energy, but they transfer momentum easily to the bottles. [73.1% picked]
Why: To transfer momentum to a bottle, the ring only has to push on the bottle for a period of time. But to transfer energy to the bottle, the ring must push on the bottle as the bottle moves in the direction of that push. Since the glass bottles are rigid and immobile, the rings just can't do work on them.
Problem 6:
You are building a decorative mobile and the first thing you do is hang a long stiff rod from a string attached to the ceiling. When you have properly centered the string on the rod, the rod experiences zero torque about its center of mass. With zero net torque acting on it, the rod
Answer: (D) experiences zero angular acceleration. [66.0% picked]
Why: The rod is balanced, meaning that it is experiencing zero net torque. It therefore experiences zero angular acceleration and rotates at constant angular velocity.
Problem 7:
During your trip to the mountains, you put water on the stove to boil. You find that it boils at a lower temperature than usual because
Answer: (C) the decreased atmospheric pressure at high altitude lets bubbles of pure steam survive at lower temperatures. [47.2% picked]
Why: With less pressure to crush them, bubbles of pure steam become stable at lower temperatures. Such bubbles form and grow at less than the sea level boiling temperature of water.
Problem 8:
You are living in an old house with aged plumbing. The pressure in the water main outside the house is good and steady, but the shower spray is surprisingly weak. And when other people are running water at the same time, the shower barely sprays at all. The reason that the shower spray is so weak while other people are running water is that
Answer: (C) speeding up the water flow in the pipes causes it to turn an even larger fraction of its total energy into thermal energy through viscous effects. [20.8% picked]
Why: The faster the water flows through the narrowed pipes, the more friction and viscous effects convert its ordered energy into disordered energy. As more water is consumed each second, the water must flow faster and it thus wastes a larger fraction of its ordered energy.
Problem 9:
The glass envelope of an incandescent light bulb contains
Answer: (C) an inert gas mixture that prolongs the life of its filament. [71.1% picked]
Why: Without that gas, the bulb would be more energy efficient but its filament would die an early death.
Problem 10:
Water is flowing gently out of the end of a garden hose. You block off most of hose's opening with your thumb and now the water sprays out at high speed because you
Answer: (D) have slowed the water flow through hose so that it wastes less energy doing working against viscous forces. [54.3% picked]
Why: By slowing the water flow, you have allowed the water to retain more of its total ordered energy.
Problem 11:
While an airplane is flying at constant velocity, the average air pressure under its wings is
Answer: (B) higher than the average air pressure above its wings. [65.5% picked]
Why: The airplane is supporting its weight by pushing the air downward. It must be generating lift, which it does by developing high pressure below its wings and low pressure above them.
Problem 12:
Heat naturally flows from a hot object to a cold object because
Answer: (D) that is the statistically most likely direction of heat flow. [87.3% picked]
Why: When heat flows from a hot object to a cold object, there is a net increase in entropy--the measure of disorder.
Problem 13:
You have just leapt off the high diving board and are falling. Neglecting any effects due to air, which of the following quantities can't change while you are above the water?
Answer: (A) Your angular momentum. [42.1% picked]
Why: The only force you experience is your weight and weight cannot exert a torque on you about your center of mass. Therefore, your angular momentum remains constant.
Problem 14:
Ice cubes gradually shrink in a frost-free freezer because
Answer: (A) water molecules go directly from solid ice to gaseous water vapor. [62.9% picked]
Why: This loss of water molecules by direct transformation from solid ice to gaseous steam is sublimation and is associated with freeze-drying.
Problem 15:
Your shampoo bottle is almost empty, so you tighten its cap and swing it rapidly in a circle with its cap end pointing outward. The remaining shampoo collects just inside the cap and you are able to squeeze out one more dollop. This technique works because
Answer: (D) the bottle's rapid inward acceleration leaves the shampoo behind so that it drifts toward the cap end of the bottle. [66.5% picked]
Why: The shampoo has inertia that tends to keep it doing what it was doing. By accelerating the container rapidly around it, you cause the shampoo to move relative to the container.
Problem 16:
Standing up in a canoe is a poor idea because the canoe can easily tip over. When you are seated low in the canoe, it is stable because any tip will
Answer: (B) raise its center of gravity and increase its total potential energy. [52.3% picked]
Why: To be in a stable equilibrium, an object's total potential energy must increase whenever it's disturbed from that equilibrium.
Problem 17:
An electric generating plant uses hot, high-pressure steam to power its generators. This steam engine can become more energy efficient as the temperature of its steam increases because
Answer: (D) the efficiency of any heat engine increases as the temperature between its heat source (the hot object) and its heat destination (the cold object) grows larger. [73.1% picked]
Why: The second law of thermodynamics allows some of the heat flowing from a hot object to a cold object to be diverted and converted into work. The greater the temperature difference between those two objects, the more entropy each joule of flowing heat produces and the larger the fraction of that heat that can be converted into ordered energy.
Problem 18:
You throw a curve ball that is bending toward your right as it travels away from you. It is able to bend in this manner because
Answer: (B) it is deflecting the airflow around it to your left. [82.2% picked]
Why: The spinning ball is push the air to the side and is experiencing a lift force toward the other side. In this case, the ball pushes the air to your left and the air is pushing the ball to your right.
Problem 19:
A fountain is spraying a clear stream of water in a high arc so that it falls gracefully into a distant pool. After the water has left the fountain's nozzle and while it is arcing through the open air, it
Answer: (D) converts kinetic energy into gravitational potential energy and then gravitational potential energy into kinetic energy. [90.4% picked]
Why: The water is exposed to the open air, so its surface pressure is atmospheric throughout its travels. However, as the water rises its kinetic energy transforms into gravitational potential energy and as it descends its gravitational potential energy transforms into kinetic energy.
Problem 20:
You are an Opera star and are attending a gala dinner to celebrate your accomplishments. To give the guests a thrill, you demonstrate your ability to break a wineglass with your voice. You tap the glass and listen to its vibrational pitch. To break the glass, you hold it near your mouth and sing loudly at
Answer: (A) its vibrational pitch. [65.5% picked]
Why: You must transfer energy to the glass via sympathetic vibration. By singing loudly at the glass at its resonant frequency, you gradually get it vibrating more and more vigorously until it shatters.
Problem 21:
You have thrown a stone toward the pond in front of you and it is now at the highest point in its arc. Neglecting any forces due to air, the net force on the stone
Answer: (A) points straight downward. [70.6% picked]
Why: The stone is a falling object, experiencing only its weight.
Problem 22:
A tall pine tree sways back and forth on a windy day. A bunch of squirrels decide to hold their annual convention in that tree and climb up to the top. When they do, the tree's swaying motion
Answer: (B) becomes slower; its period of motion increases. [36.5% picked]
Why: The squirrels add mass to the tree but have no effect on its stiffness. A harmonic oscillator's period depends only on its inertia and the stiffness of its restoring influences. Increasing that inertia slows the period while increasing the stiffness of its restoring influences speeds up the period.
Problem 23:
Two children are swinging side by side on identical playground swings. The heavier one weighs twice as much as the lighter one. As they swing back and forth, you notice that
Answer: (A) the two children swing with the same period. [62.9% picked]
Why: The period of a pendulum depends only on its length and on the strength of gravity.
Problem 24:
You put a glass of water on the floor of a large room full of dry air. The room is so perfectly insulated that no heat flows in or out of it. A day later, the glass is empty and the room air is moist. Compare the total entropy of the earlier situation (full glass plus dry room air) to the total entropy of the later situation (empty glass plus moist room air).
Answer: (D) The total entropy of the later situation is higher than the entropy of the earlier situation. [70.1% picked]
Why: The isolated system cannot experience a decrease in entropy; that would violate the second law of thermodynamics. In fact, the entropy increases because the separated water and dry air is more orderly than the mixed system.
Problem 25:
A kiln is used to fire pottery—to heat it red or even yellow hot so that it fuses into a hard, finished ceramic. You have four pots to fire, each with its own surface color. One is black, one is white, one is shiny aluminum-like, and one is essentially transparent. You place the four pots in a kiln and heat them to the same yellow-hot temperature. When you look inside the kiln, the one that is glowing most brightly is the one that is
Answer: (B) black. [95.4% picked]
Why: A black object is good at interacting with light, whether it is absorbing light (which it does perfectly) or emitting thermal radiation (which it does ideally well).
Problem 26:
To turn left, an airplane dips its left wing slightly. By tipping in that manner, the airplane is using
Answer: (D) its aerodynamic lift to push it to the left. [75.1% picked]
Why: By tiling its wings, the plane is managing to deflect the passing air to the right and the air pushes the plane to the left. The force that the air exerts on the plane is a lift force.
Problem 27:
On a hot, humid day, your moist, sweaty skin doesn't do a very good job of cooling you off. If the air were less humid, you'd cool off better because water molecules would
Answer: (D) land on your moist skin less often. [15.2% picked]
Why: Evaporation is fastest when molecules leave the liquid for the gas far more often than they land from the gas on the liquid. On a dry day, the low landing rate makes evaporation rapid.
Problem 28:
You are rearranging the furniture and are moving your desk to the other side of the room. To keep your desk sliding across the level floor at constant velocity, you have to keep pushing it forward. The net force on the desk is
Answer: (D) zero, even though the ground is exerting a backward frictional force on the desk. [59.9% picked]
Why: The desk is coasting at constant velocity, so the net force on it must be zero.
Problem 29:
You are sitting still in your airplane seat, holding your mp3 player in your hand, when the airplane begins to rise to a higher altitude. As the plane rises, you are
Answer: (B) doing positive work on your mp3 player. [93.9% picked]
Why: You are supporting the mp3 player by pushing it upward and the mp3 player is moving upward, so you are doing work on it.
Problem 30:
You're standing on the middle of the Ruffner Hall pedestrian bridge when a group of prospective students jumps up and down and sets the bridge bouncing. As you bounce up and down about equilibrium, your acceleration is greatest when you are
Answer: (C) at the top and bottom of the bounce. [60.4% picked]
Why: Your accelerate is at its greatest when the net force on you is at its greatest. Since you're part of a harmonic oscillator, the restoring force you're experiencing increases with displacement from equilibrium. At your peak displacements, you are experiencing the peak forces and therefore the peak accelerations.
Problem 31:
Two marbles are in a large bowl and the system is completely isolated from its surroundings. Each of the four arrangements listed below as possible answers contains exactly the same amount of total energy, although that energy may be in different forms in the different arrangements. Which arrangement has the most entropy?
Answer: (C) The marbles resting motionless in the bottom of the bowl. [80.2% picked]
Why: The final state of a thermal isolated system must be the one with the most entropy. Since the marbles eventually end up resting motionless at the bottom of the bowl, that's the state with the greatest entropy.
Problem 32:
When you strike a match on the side of the match box, you are using sliding friction to
Answer: (B) provide the activation energy needed to initiate the chemical reactions of combustion. [95.9% picked]
Why: The match has the stored chemical potential energy it needs to burn, but something must help initiate the reactions. Sliding friction provides the initial thermal energy needed to start the reactions and get the match burning.
Problem 33:
It's hard to stand a coin on its edge. However, if you roll a coin forward on its edge, it stays up easily. The rolling coin is
Answer: (A) automatically steering itself under its center of gravity and is thus dynamically stabilized. [87.3% picked]
Why: Like the wheel of a bicycle, the coin is using its motion to return it to its unstable upright equilibrium. In this case, the effect is probably a gyroscopic one: the spinning coin precesses in such a way that it steers under its own center of gravity and stabilizes itself.
Problem 34:
The houseware store you have just entered has a uniform temperature throughout. You pick up a heavy copper pot and place it on a soft, fabric mat. As a result of this contact, heat
Answer: (A) does not flow at all. [80.7% picked]
Why: The two objects are at the same temperature and are therefore in thermal equilibrium. Heat does not flow between objects that are in thermal equilibrium.
Problem 35:
Two people have just bungee-jumped simultaneously and are now bouncing gently beside one another. The heavier person weighs twice as much as the lighter person and is hanging from two standard bungee cords while the lighter person is hanging from just one standard cord. As the two people bounce, they notice that
Answer: (D) they are bouncing equally often. [63.5% picked]
Why: The heavier person hanging from two bungee cords is equivalent to two lighter people hanging from one cord each and holding hands. The pair of hand-holding lighter people and the single heavier person must bounce with the same period because they're equivalent systems. And whether or not the people hold hands should and won't affect their periods either.
Problem 36:
You are a passenger in a car that is making a sharp left turn. A book on the dashboard begins sliding toward the right because
Answer: (A) inertia trying to make it continue straight ahead as the car moves leftward. [95.4% picked]
Why: The car is effectively driving out from under the book.
Problem 37:
Compared to a smooth golf ball, a dimpled golf ball produces
Answer: (C) a smaller turbulent wake behind it as it flies. [71.1% picked]
Why: The dimples cause the ball to develop a turbulent boundary layer, which in turn helps the main airflow remain attached to the back of the ball longer. The resulting wake is smaller and the ball experiences less pressure drag.
Problem 38:
You're blowing across the mouth of a bottle and it's whistling loudly with a clear tone. If you replace the air in the bottle with helium and again blow across the mouth of the bottle, its tone will be
Answer: (D) higher in pitch, but will quickly return to normal as the helium escapes. [93.9% picked]
Why: Helium has a lower density than air and accelerates faster in response to the same forces. Since the stiffness of the gas column in the bottle doesn't change when the air is replaced with helium, the column acts as a harmonic oscillator with less mass but the same restoring forces as before and it vibrates at a higher pitch.
Problem 39:
When your home air conditioner is working, the air in your room is being cooled by passing it by
Answer: (B) an evaporator, in which a working fluid is turning from a liquid to a gas. [57.4% picked]
Why: As the working fluid evaporates, it needs its latent heat of evaporation and it therefore absorbs heat from its surroundings.
Problem 40:
Your home is heated by a heat pump that transports heat from the outdoor air to the indoor air. As this device operates,
Answer: (A) it delivers more heat to the indoor air than it removes from the outdoor air. [50.3% picked]
Why: The heat pump delivers to the indoor air not only the heat it extracts from the outdoor air, but also the thermal energy it produces from the ordered energy that powers it.
Problem 41:
In a spaceship that is orbiting the earth with its rockets off, one of the mechanisms of heat transfer no longer works well:
Answer: (B) convection. [65.5% picked]
Why: Without gravity, buoyancy is absent, so heated air is no longer buoyant and convection no longer works. As for the US postal service, it has never been much of a heat transfer mechanism.
Problem 42:
You are trapped in an elevator car that is out of control. It won't stop moving for more than an instant and is going up and down randomly. At this moment, you are feeling extremely heavy, so you must be
Answer: (B) accelerating upward. [80.7% picked]
Why: You experience a weight-like "feeling of acceleration" in the direction opposite your acceleration. To feel extra heavy downward, you must be accelerating upward.
Problem 43:
An empty glass bottle with its cap screwed on tightly is floating on a still lake. The bottle's weight is equal to the weight of
Answer: (D) the mixture of water and air that it is displacing. [62.4% picked]
Why: Since the bottle is motionless, the buoyant force on the bottle must balance its weight. That buoyant force is equal to the weight of exactly what it is displacing, which in this case is both water and air.
Problem 44:
As you push a child on a swing, you can increase the amplitude of the motion by pushing the child away from you whenever she is
Answer: (B) moving away from you. [68.5% picked]
Why: To increase her amplitude of motion, you must invest more energy in her swing by doing work on her. That means that you must push her away as she moves away.
Problem 45:
You are weighing yourself on a bathroom scale. It reads your correct weight up until the moment you jump off the scale. For that brief time, the scale reads heavier than your weight because the scale is pushing up on you with a force
Answer: (C) greater than your weight and you are accelerating upward. [39.1% picked]
Why: As you jump, you push down on the scale extra hard and it responds by pushing up on you equally hard in the opposite direction. As a consequence of that large upward force, you accelerate upward and soon rise up into the air.
Problem 46:
Water is sloshing back and forth gently in your aquarium. You shake the aquarium until the water is sloshing twice as strongly. The period of the slosh—the time it takes to complete a full cycle of motion—is now
Answer: (D) the same as it was earlier. [61.9% picked]
Why: The sloshing water is a harmonic oscillator and its period of oscillation is independent of the amplitude of that oscillation.
Problem 47:
You're floating on a raft in the open ocean. As the waves move forward underneath the raft, you move
Answer: (D) in a circle, without making any net progress forward. [80.7% picked]
Why: The wave is merely a disturbance of the water's surface. Although this disturbance moves across the water, the water itself doesn't go anywhere overall. It simply circles in place as the wave passes.
Problem 48:
While an airplane is flying at constant velocity, the downward momentum it transfers to the passing air each second is
Answer: (B) equal to the downward momentum transferred to it by gravity each second. [56.9% picked]
Why: The plane is giving all of the momentum it receives from gravity to the air and thus not accumulating any.
Problem 49:
The maximum speed of a rocket-propelled spaceship with an exhaust speed of 5000 mph is
Answer: (C) limited only by the ratio of fuel to spaceship. [59.9% picked]
Why: As long as the spaceship can keep ejecting fuel out the exhaust, it can keep accelerating forward.
Problem 50:
Compared to regular gasoline, premium or high-octane gasoline
Answer: (A) is harder to ignite and therefore more resistant to knocking. [82.2% picked]
Why: High-test fuels are harder to ignite and therefore better suited to high compression engines.
Problem 51:
You turn on the refrigerator in your dorm room and it begins cooling the food inside it. The refrigerator cools the food by
Answer: (C) using electricity to pump the food's thermal energy into your dorm room. [78.7% picked]
Why: The refrigerator can't make the food's thermal energy disappear, so the best it can do is transfer that energy elsewhere. It extracts thermal energy from the food and releases that thermal energy into the room air along with thermal energy produced from electrical energy during the pumping process.
Problem 52:
A spacecraft in deep space is coasting directly toward Sirius, the dog star. To change its course, it fires its rocket thruster at right angles to its direction of travel. After this procedure, the total momentum of the spacecraft plus its original fuel now points
Answer: (D) directly toward Sirius. [16.8% picked]
Why: Although a rocket can redistribute momentum between its various pieces, including its ship and fuel, it can't create or destroy momentum. This rocket sends fuel in one direction and the ship in another, but its overall momentum remains the same and continues to point directly toward Sirius.
Problem 53:
It's a cold day and you are standing just inside the front door of your house. A guest opens the door so that you can see the neighbors' houses. Before any cold air reaches you, you already begin to feel colder because
Answer: (B) the cold outdoor objects radiate less heat toward you than the warm front door did. [36.0% picked]
Why: Prior to opening the door, the door was radiating heat at you consistent with its warm temperature. Once the door is open, you no longer have that source of heat radiating at you. Instead, you have the cooler outside radiating at you and it radiates far less heat.
Problem 54:
Firefighters are battling a fire on the 10th floor of an apartment building. When they stand on the ground, their fire hose can only shoot the water up to the 8th floor. So they carry the end of the same fire hose up a 2-story-tall ladder and shoot water upward. Now the water rises to the
Answer: (B) 8th floor. [77.7% picked]
Why: Whether they shoot water all the way up or let it rise part way inside a hose, the water still has only enough total energy to reach the 8th floor.
Problem 55:
During the compression stroke in an internal combustion engine, the piston squeezes the mixture of fuel and air into a small volume inside the cylinder. As the piston moves into the cylinder,
Answer: (D) both the pressure and the temperature of the fuel and air mixture increase. [90.9% picked]
Why: The piston is doing work on the gas inside the cylinder and its temperature is rising. Since both its density and its temperature are increasing, the gas's pressure surges upward.
Problem 56:
To propel itself forward, a rocket pushes against
Answer: (A) its own exhaust. [92.4% picked]
Why: The only thing the rocket has to push against is what it has onboard. It pushes its own fuel backward and its fuel responds by pushing it forward.
Problem 57:
After its rider has fallen off it, a skateboard rolls directly up a ramp, comes to a stop, and rolls back down. During its travel along the ramp, the skateboard accelerates
Answer: (B) steadily downhill. [66.0% picked]
Why: Throughout the skateboarder's trip along the ramp, the net force on the skateboarder is downhill and the skateboarder accelerates downhill.
Problem 58:
The air leaving a jet engine has
Answer: (A) more backward momentum than it had when it entered the jet engine. [80.2% picked]
Why: The jet engine propels the plane by giving the air passing through it backward momentum and obtaining forward momentum as a result.
Problem 59:
When you operate the broiler of your electric oven, only the heating element at the top of the oven becomes hot. As you broil vegetables in the middle of the oven, those vegetables are heated primarily by
Answer: (D) radiation. [67.0% picked]
Why: Air is a poor conductor of heat and natural convection doesn't work when the hottest object is on top. All that remains is radiative heating.
Problem 60:
Your bottle of oil and vinegar salad dressing has separated, so that the oil is floating on the vinegar. You notice that a large herb is floating motionless at the interface between the oil and the vinegar. That herb's average density is
Answer: (D) less than that of vinegar but more than that of oil. [76.1% picked]
Why: For the herb to float on the vinegar, its density must be less than that of the vinegar. For it to sink in the oil, its density must be more than that of the oil.
Problem 61:
Tightening a violin string
Answer: (D) stiffens the string's restoring forces and increases the frequencies of its vibrational modes. [94.9% picked]
Why: Like all harmonic oscillators, a violin string's frequency depends on a inertial aspect and a stiffness aspect. Tightening the string boosts the stiffness aspect and thereby increases all the frequencies of the violin string's modes.
Problem 62:
As a large wave approaches the beach, its crest suddenly dives over the water in front of it and breaks up into white froth. This diving surf occurs when
Answer: (D) the water has become too shallow to form a complete wave crest. [69.0% picked]
Why: Ocean waves are produced from local water and when the water becomes too shallow to form crests, they break. In diving surf, the steep seabed spoils the front end of the crest so that only the back end of the crest forms and dives over the missing front end.
Problem 63:
You're hanging clothes on a clothesline that is strung tightly between two posts and you notice that the clothesline tends to vibrate up and down in its fundamental mode. As you add clothes to the clothesline, the period of that fundamental vibration
Answer: (A) increases. [35.0% picked]
Why: The tension in the taut clothesline doesn't change significantly when you add more clothes, so their primary influence is to add mass to the system. By increasing the inertial aspect of this harmonic oscillator, you slow down its vibrations and increase the period.
Problem 64:
Your hanging birdfeeder swings gently back and forth like a pendulum. If a gust of wind starts it swing with twice its current amplitude, the period of its swing will
Answer: (B) remain unchanged. [87.3% picked]
Why: The birdfeeder is a harmonic oscillator, with a restoring force that is almost exactly proportional to displacement from a stable equilibrium. As a result, its period of motion doesn't depend on the amplitude of that motion.
Problem 65:
An operating internal combustion engine can't avoid releasing some waste heat into its environment because
Answer: (B) converting all of the heat into work would violate the laws of thermodynamics. [68.5% picked]
Why: Like all heat engines, the internal combustion engine can only convert part of the heat leaving its hot burned gases into work. It must allow the rest of the heat to flow into the cold outdoor air.
Problem 66:
Your friend has purchased a thin cooking pot made entirely of stainless steel. Your friend places this pot on the coil burner of an electric stove and tries to cook risotto. To your friend's dismay, the risotto cooks very unevenly and burns at various places on the bottom of the pot. The pot's problem is that stainless steel is a
Answer: (B) poor conductor of both electricity and heat. [46.2% picked]
Why: Stainless steel, like many alloys, isn't good at conducting electricity. And since the same electrons that carry electricity through metals are responsible for carrying most of the heat through metals, stainless steel is also a mediocre thermal conductor.
Problem 67:
When an airplane lifts its nose too high while flying forward,
Answer: (B) the airstream separates from the tops of its wings and the airplane experiences less lift and more drag. [62.4% picked]
Why: When the wings are tipped to too great an angle of attack, the airflow over the top of the wings stalls and the plane develops a turbulent wake.
Problem 68:
The "A" string of a guitar normally vibrates 440 times per second in its fundamental mode. If you hold the middle of that string so that it can't move and pluck one of the half-strings, its main vibration will occur at
Answer: (D) 880 times per second. [76.6% picked]
Why: The two half-strings that you create by holding the middle of the string are half the length of the whole string and therefore vibrate at the second harmonic frequency -- twice the fundamental frequency.
Problem 69:
You are bouncing gently on a springboard before diving into the water. While your feet remain on the board's surface, the period of your bounce is independent of the height of that bounce. But once your feet start leaving the board, the period of your bounce increases because
Answer: (C) the restoring force pushing you back toward equilibrium is no longer proportional to your displacement from equilibrium; it is too weak while you are in the air. [68.5% picked]
Why: Once you leave the surface of the springboard, you're no longer a harmonic oscillator. Above the board, the restoring force becomes your weight and that's constant and independent of height.
Problem 70:
Your desk lamp has a dimmer switch so that you can reduce the power going to its incandescent bulb. If you reduce the power going to the bulb by 50%, it will emit
Answer: (A) less than 50% as much visible light as before. [21.8% picked]
Why: The filament will be too cool to emit much visible light. While the bulb will continue to consume 50% of its usual power, most of the heat it will emit will be useless and you'll get less than 50% of the original visible light out of it.
Problem 71:
Many hospitals measure patients' temperatures by inserting optical devices into their ears. Those devices sense temperature with light by
Answer: (C) looking at the spectrum of wavelengths emitted by the ear canal. Shorter wavelengths mean a hotter ear. [50.3% picked]
Why: An ear behaves as a nearly perfect black body: light entering it is essentially perfectly absorbed and it is an essentially perfect emitter of thermal radiation. Like all black bodies, it emits a spectrum of light that is characteristic of its temperature.
Problem 72:
Your glass cup contains a mixture of ice and water. It's a warm day and heat is flowing into that glass. As the heat enters the mixture of ice and water, the temperature of the mixture
Answer: (B) remains constant, but the ice gradually converts to liquid water. [69.5% picked]
Why: Liquid and solid water can coexist in equilibrium only at the melting point of water. Adding heat simply transforms more of the ice into liquid water.
Problem 73:
You and your best friend are trying to pull one another toward your respective dorm rooms. You're the stronger of the two and, with a mighty tug, you drag your friend into your room. As you are pulling your friend toward your room, the force you exert on your friend is
Answer: (B) equal in amount to the force your friend exerts on you. [90.4% picked]
Why: The two of you pull equally hard on one another, as recognized by Newton's third law of motion.
Problem 74:
The plastic tube carrying water to a fountain has a bulge. As the rapidly flowing water in the tube enters this unusually wide part of the tube, it
Answer: (D) slows down and its pressure rises. [39.1% picked]
Why: The bulge acts as the opposite of a nozzle, allowing the water to slow down and experience a rise in pressure.
Problem 75:
Compared to the filament of a 50 watt light bulb, the filament of a 100 watt light bulb operates at
Answer: (A) the same temperature but has twice as much surface area. [24.9% picked]
Why: The 100 W filament emits the same spectrum of light as the 50 watt filament, so their temperatures must be the same. To maintain its heat balance, the 100 W filament must be twice as effective at emitting radiation and it does this by having twice the surface area.
Problem 76:
If the earth's gravity became stronger, waves on the open ocean would travel
Answer: (D) faster than with normal gravity. [13.7% picked]
Why: Like all harmonic oscillators, water surface waves have an inertial aspect and a stiffness aspect. Gravity is responsible for the stiffness of the restoring force, so increasing its strength increases the stiffness of that restoring force. Everything cycles faster and the waves travel faster as a result.
Problem 77:
You add 1 joule of heat to a cold object and 1 joule of heat to a hot object. As a result of these additions,
Answer: (A) the cold object experiences a greater rise in entropy than the hot object. [76.1% picked]
Why: Added heat a unit of heat is more disordering to a colder object (which is relatively ordered) than to a warmer object (which is already relatively disordered).
Problem 78:
You live on a small island and it is now high tide. Your friend lives on another small island, exactly on the opposite side of the earth from your island. On your friend's island, it is now
Answer: (B) high tide. [79.2% picked]
Why: You are both in the midst of tidal bulges -- one on the side of the earth nearest the moon and one on the side of the earth farthest from the moon.
Problem 79:
If you use a microwave oven to heat clean water in a smooth glass container, you can sometimes superheat the water—that is, raise its temperature well above water's boiling temperature without it boiling. In that case, the very hot water fails to boil because
Answer: (A) even though bubbles of pure steam are stable at such high temperatures, they cannot form without help. [84.3% picked]
Why: Just because bubbles of pure steam can form in hot water doesn't mean that they'll get started. They need something to help them form.
Problem 80:
You drop a can of soup 10 feet onto a cement floor and it hits squarely on the bottom end of that can. The impact is likely to
Answer: (A) make the sides of the can bulge outward near the bottom end of the can. [32.0% picked]
Why: The sudden transfer of momentum from the can to the floor involved a huge force and that force was exerted by an enormous pressure in the lower part of the can. This pressure surge, essentially water hammer, also dented the sides of the can outward near its bottom.