Physics 105 - How Things Work - Fall, 1999

Midterm Examination


Given Wednesday, October 13, from 1:00 PM to 1:50 PM




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 the midterm examination.

Problem 1:

When you are out fishing on a lake one day, you get your line snagged on a floating tree branch. Luckily the branch is not attached to anything, so you can reel it in to unhook your line. As you reel in the branch, it moves toward you at a constant speed. The amount of work you are doing on the branch is

(A) exactly zero.

(B) constant, but negative—the branch does work on you.

(C) constant and positive—you do work on the branch.

(D) exactly equal to the kinetic energy of the branch.

Problem 2:

Your pet hamster has an exercise wheel in his cage. He can climb into this wire wheel and then run in it as though he were on a treadmill. When he is running in the wheel, he remains stationary at the bottom of the wheel while it spins around him. In this case his angular velocity is

(A) equal in magnitude to the wheel's angular velocity, but in the opposite direction.

(B) zero.

(C) increasing as he does work on the wheel.

(D) equal in both size and direction to the angular velocity of the wheel.

Problem 3:

The county fair has a game that supposedly measures your strength. When you pound on one end of a lever with a giant hammer, the lever tosses a metal ball up a tall, vertical track toward a bell located 10 meters above you (see Figure E). You decide to give it a try and, to your delight, manage to hit the bell on the first try. As the ball rises upward on the frictionless track, it experiences

(A) a constant upward force that depends only on the momentum of the ball as it rises toward the bell.

(B) an upward force that decreases steadily but yet remains substantially greater than zero even when the ball hits the bell.

(C) an upward force that decreases steadily and reaches zero exactly at the moment the ball hits the bell.

(D) no upward forces.

Problem 4:

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 average density of the helium-filled bag decreases even though the buoyant force on the bag remains constant as it fills.

(C) as the volume of displaced air increases, the buoyant force increases until it is larger than the weight of the helium-filled bag.

(D) the helium-filled bag's weight decreases as you put more lightweight helium particles inside it.

Problem 5:

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 shower head because faster moving water

(A) has less pressure than slower moving water.

(B) has less kinetic energy than slower moving water.

(C) has less gravitational potential energy than slower moving water.

(D) losses more energy to viscous drag as it flows through the delivery pipes.

Problem 6:

As you ride on a merry-go-round, you feel a strong outward pull that feels just like the force of gravity. This fictitious force occurs because

(A) your velocity is toward the center of the merry-go-round and you experience a fictitious force in the direction opposite your velocity.

(B) you are accelerating away from the center of the merry-go-round and experience a fictitious force in the direction of your acceleration.

(C) you are accelerating toward the center of the merry-go-round and experience a fictitious force in the direction opposite your acceleration.

(D) your velocity is away from the center of the merry-go-round and you experience a fictitious force in the direction of your velocity.

Problem 7:

Water in a smooth, gentle river flows past the cylindrical post supporting a dock. At the point where this water first encounters the vertical post (see Figure A), the water level

(A) is the same as elsewhere on the river.

(B) rises slightly higher than elsewhere on the river.

(C) drops slightly lower than elsewhere on the river.

(D) vibrates up and down, but averages to the same height as elsewhere on the river.

Problem 8:

You are out in space, so far from any star or planet that gravity is insignificant. You throw two rubber balls so that they drift forward as a pair. These balls continue to touch one another with one ball directly in front of the other. Which of the balls is pushing on the other?

(A) The ball in front is pushing backward on the ball behind and the ball behind is pushing forward on the ball in front.

(B) Only the ball behind is pushing forward on the ball in front.

(C) Only the ball in front is pushing backward on the ball behind.

(D) Neither ball is pushing on the other.

Problem 9:

When you blow on a pinwheel, it starts to spin. Even when you aim the air directly toward the wheel's pivot (see Figure C), you produce a torque on the wing-like blades and they undergo angular acceleration. This torque is produced by

(A) lift forces on the pinwheel's blades.

(B) a deflection of the air stream directly away from the pivot.

(C) the buoyant force due to the high pressure air hitting the blades of the pinwheel.

(D) drag forces on the pinwheel's blades.

Problem 10:

You've always wondered how much one of your friends weighs and devise a scheme to measure his weight secretly. You have him sit in a tubular steel chair. This popular style of chair (see Figure F) consists of a single steel tube that's bent into a frame and that supports a seat bottom and a back. The empty chair weighs 10 pounds and is 30 inches tall. The frame acts as a spring and bends downward slightly when the chair is occupied. When you sit properly in the chair yourself, it bends downward 1 inch. When your friend sits properly in the chair, it bends downward 2 inches. From that observation, you know that your friend weighs about

(A) 300 pounds.

(B) twice as much as you do.

(C) four times as much as you do.

(D) 150 pounds.

Problem 11:

When playing hockey, you flick the puck across the ice rink with your stick. The puck then moves across the frictionless surface of the ice at constant velocity. As the puck moves across the ice, its kinetic energy is

(A) constant because kinetic energy is a conserved quantity.

(B) zero because it isn't accelerating.

(C) increasing as the ice does work on it to support it against the force of gravity.

(D) constant because its speed is constant.

Problem 12:

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) float at the same height as before the air left.

(B) move downward slightly and float somewhat lower (deeper) in the water.

(C) sink to the bottom of the water.

(D) move upward slightly and float somewhat higher (less deep) in the water.

Problem 13:

You are jumping up and down on a trampoline, under the watchful supervision of your gymnastics coach. (We've included the coach because trampolines are dangerous and we don't want to be sued for any virtual injuries that might occur during this exam.) Each time you bounce off the trampoline, you stretch its surface and springs. Just before the surface and springs reach their maximum stretch, your velocity is

(A) downward but your acceleration is upward.

(B) downward and your acceleration is downward.

(C) upward but your acceleration is downward.

(D) upward and your acceleration is upward.

Problem 14:

As you stand at the end of a diving board, the board bends downward. While you slowly bend your knees and prepare to jump, the board doesn't move. Finally, when you begin to jump upward, the board

(A) moves upward since the board's restoring force is in the direction of its unbent equilibrium position.

(B) moves downward since you must apply a downward force on the board larger than your weight in order to jump upward.

(C) moves upward since your weight is no longer there to hold the board down.

(D) stays in the same position since the restoring force provided by the spring board balances your weight.

Problem 15:

When you pour honey into a bowl, it flows smoothly. If you did the same with water, it would splash. These different behaviors occur because honey's high

(A) viscosity keeps it flowing smoothly while water's low viscosity allows inertia to break its flow into many separate pieces.

(B) momentum keeps it moving in a straight line while water's low momentum allows it to turn abruptly in many different directions.

(C) density keeps it flowing smoothly while water's low density allows it to float upward and splash about.

(D) mass keeps it flowing smoothly while water's low mass allows it to acquire lots of angular momentum.

Problem 16:

Your roommate's car has a pair of "fuzzy dice" hanging by a string from its rearview mirror. Yes, they're tacky but you don't have the heart to say anything. Anyway, these dice do a nifty job of indicating how the car is moving. For example, if the dice swing forward toward the car's windshield while the car is on a level road, you know that the car is

(A) accelerating backward.

(B) accelerating forward.

(C) traveling backward at a steady pace.

(D) traveling forward at a steady pace.

Problem 17:

A hot air balloon is open at the bottom and the passengers in the basket beneath can see the inside of the balloon's envelope or "skin." Hot air doesn't flow out of the balloon's open bottom because

(A) viscous forces slow the flow of air out of the balloon.

(B) the balloon's canvas envelope pulls outward, sucking air into the balloon.

(C) the air inside that open bottom is at atmospheric pressure.

(D) the flame pushes the air up into the balloon through that open bottom.

Problem 18:

You are holding two identical-looking balloons, one filled with air and one filled with water. You drop these two balloon from a very tall bridge and notice that the water-filled balloon hits the ground first because its terminal velocity is larger. The terminal velocity of the water balloon is larger than that of air balloon because

(A) conservation of momentum requires the lighter air-filled balloon to travel more slowly.

(B) although the drag forces on the two equally shaped balloons are the same, the buoyant force on the air balloon is larger so the net force on that balloon is smaller and it falls more slowly.

(C) the larger force of gravity on the water balloon must be balanced by a larger drag force, which occurs at a higher speed.

(D) although the force of gravity on the two balloons is the same, the water balloon has more inertia and travels downward more quickly.

Problem 19:

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 you hand is

(A) down and away from you.

(B) straight down.

(C) zero.

(D) up and away from you.

Problem 20:

You are swinging a ball on a string around in a horizontal circle above your head and it moves in that circle at a constant speed. We are neglecting air resistance and gravity. Even though the string is exerting a force on the ball, the ball's energy isn't changing because

(A) the negative amount of work that the centripetal force does on the ball is exact cancelled by the positive amount of work that the centrifugal force does on it.

(B) the positive amount of work that the centripetal force does on the ball is exact cancelled by the negative amount of work that the centrifugal force does on it.

(C) the ball's inertia is conserved.

(D) the centripetal force the string exerts on the ball is exactly at right angles to the ball's motion.

Problem 21:

A squirt gun is a simple type of water pump in which a plunger attached to the trigger forces water out of a nozzle and across the room. When you squeeze the trigger of the gun, water squirts out of the nozzle because

(A) the pressure inside the gun is higher than atmospheric pressure.

(B) the water is compressed at the plunger so it must expand out the nozzle.

(C) the Bernoulli effect causes the pressure of water leaving the nozzle to be less than atmospheric pressure.

(D) the Bernoulli effect causes the water's overall energy to increase as it travels through the narrow nozzle.

Problem 22:

When a modern car crashes into a tree and comes to an abrupt stop, the driver's face and chest collide with an air bag rather than with the steering wheel. The driver's chances of serious injury are reduced by hitting the air bag rather than the steering wheel because the driver transfers

(A) more momentum to the air bag than he would to the steering wheel if there were no air bag.

(B) the same amount of momentum to the air bag as he would to the steering wheel if there were no air bag, but he does so with a larger force because of the air bag.

(C) the same amount of momentum to the air bag as he would to the steering wheel if there were no air bag, but he does so with a smaller force because of the air bag.

(D) less momentum to the air bag than he would to the steering wheel if there were no air bag.

Problem 23:

You are trying to deliver water to a sink in the tree house in your backyard. You run an old hose from the spigot behind your home, across your yard, and up the tree to the tree house (see Figure B). You let water fill the hose all the way to the tree house sink and then leave the hose pressurized overnight, with no water flowing in it. Unfortunately, the hose can't tolerate high pressure anymore and it springs a leak around midnight. By morning your whole backyard is a swamp. The most likely site for the leak is

(A) on the way up the tree—the most vertical portion of the hose.

(B) on the ground—the lowest point on the hose.

(C) at the spigot—the first point on the hose.

(D) at the sink—the highest point on the hose.

Problem 24:

You're filling in for Wily Coyote in a Roadrunner cartoon. You're trying to tip a huge boulder off a cliff and onto the poor bird. You have just enough strength to start the boulder rocking rhythmically back and forth. To make the boulder rock farther and farther, you should only push it forward when it's

(A) on the far side of its equilibrium position.

(B) rocking away from you.

(C) rocking toward you.

(D) on your side of its equilibrium position.

Problem 25:

To set the world land speed record and travel faster than the speed of sound, the Thrust SSC vehicle used two jet engines that produced about a 250,000 horsepower. The principal reason why this car needed so much power to travel so fast is that

(A) a car's momentum is proportional to its power, so reaching very high momentum requires very high power.

(B) the pressure drag on a car increases dramatically as the car's speed increases.

(C) the force of the car's momentum was enormous and the jet engines were needed to supply that force.

(D) Newton's second law requires a very fast moving object to have a very large acceleration and thus a very large force.



Please give a brief answer in the space provided. Part II is worth 33% of the grade on the midterm examination.

Problem 1:

You are riding your bicycle over a large hill. You have just reached the top and as you start down the other side, you stop pedaling and begin to coast. It's a long, steep hill with a constant downward slope. You are riding a superbly engineered bicycle, which wastes no energy in sliding friction.

(A) As you coast down the hill, your velocity increases. What is the direction of your acceleration?

(B) At this point, how does the magnitude of your apparent weight compare to that of your actual weight?

(C) If you continue to coast down the hill, you will eventually stop accelerating and settle in at a constant downhill speed. Even though you are not accelerating, there are still three forces acting on you that perfectly balance each other. What are these three forces?

(D) In what direction do each of these three forces act?

Problem 2:

It's autumn and you are clearing leaves from your yard with a leaf blower. A stream of air rushes out of the blower's nozzle and blows the leaves across the yard.

(A) After the stream of air leaves the nozzle but before it encounters any leaves, what is the pressure of the air stream? (Use words like "more, less, equal, zero" rather than numbers and units.)

(B) When the stream of air encounters a leaf, what is the pressure of the air stream at the surface of that leaf? (Again, we're looking for comparisons rather than exact values other than zero.)

(C) You and the blower are transferring forward momentum to the air all the time, yet your momentum remains essentially constant. Why doesn't your momentum change very much?

(D) You're tired of chasing leaves so you begin blowing air across other objects. When you blow a stream of air across the domed lid of a garbage can (see Figure D), that lid suddenly pops upward and then blows away. Evidently, the air stream "sucked" the lid off the can! Use acceleration to explain briefly why the air pressure above the lid dropped below atmospheric pressure. A single, focused sentence is all that's expected here.

Problem 3:

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 (see Figure G). 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?

(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?

(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?

(D) If the only person on the escalators is riding down from the second floor toward the first floor, energy is being transferred from what to what?