Midterm Exam

Physics 105 - How Things Work - Fall, 1995

Midterm Examination

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 the midterm examination.

Problem 1:

A bean bag will bounce higher when you drop it onto a soft rubber surface than it will when you drop it onto a hard rubber surface. That's because

(A) the soft rubber deforms more as it slows the falling bean bag and it stores more of the collision energy.

(B) the bean bag has more momentum when it hits the soft rubber than when it hits the hard rubber.

(D) the hard rubber exerts less torque on the bean bag.

Problem 2:

You are trying to knock over a stack of weighted bottles at the state fair. You can throw either a super ball or a bean bag. Since both objects have identical masses, the most effective choice is

(A) the bean bag because it transfers the most momentum to the bottles when it hits and stops.

(B) the super ball because it transfers the most momentum to the bottles when it hits and rebounds.

(D) the bean bag because it weighs more than the super ball.

Problem 3:

The city water pipes enter your home on the ground floor. Pipes inside your home then carry water up to the second and third floors and down to the basement. The water pressure in your home is highest

(A) on the third floor.

(B) on the second floor.

(D) in the basement.

Problem 4:

You reach out to push on a car as it passes by and manage to exert a forward force of 10 N on it. When you do this, the car exerts

(A) a backward force of somewhat more than 10 N on you, because, in addition to the reaction force, it must accelerate your hand backward.

(B) a backward force of 10 N on you, because forces always come in equal but oppositely directed pairs.

(C) a backward force of somewhat less than 10 N on you, because the car's forward velocity reduces the force it needs to accelerate forward.

(D) no backward force on you at all, because the force of its velocity is already enough to keep it moving forward.

Problem 5:

If you drop a golf ball and a bowling ball simultaneously from roof of your home, they will both hit the ground at the same moment. The two balls travel downward side-by-side because gravity gives them identical

(A) downward accelerations.

(B) downward momenta.

(D) weights.

Problem 6:

You're bicycling swiftly down a hill, traveling in a straight line at a steady speed. The wind is blowing in your face and you're pedaling furiously. The net force on you is

(A) downhill.

(B) uphill.

(D) zero.

Problem 7:

You have been running track races in smooth-soled shoes. During each start, you have been wasting 100 joules of energy as thermal energy because of friction between your shoes and the track. To help this situation, you purchase a pair of spiked shoes. Now when you start a race, the frictional force your feet experience from the track is increased by a factor of 5 and the shoes do not slide across the track at all. During each start, the amount of energy you now waste as thermal energy because of friction between your spiked shoes and the track is

(A) 4 joules.

(B) 20 joules.

(D) 500 joules.

Problem 8:

A gymnast doing a double back flip leaps off the floor with her arms and legs extended and then pulls herself into a very compact position. In her compact shape, she rotates very rapidly and completes two full rotations before opening back up to land on the floor. During the time that she is not touching the floor, the one aspect of her motion that is constant is her

(A) momentum.

(B) angular momentum.

(D) angular velocity.

Problem 9:

When you close a water faucet, the water that was about to flow out of the faucet

(A) remains at the faucet's inlet, but the water pressure in the pipe drops to atmospheric pressure.

(B) remains pressed against the faucet's inlet by the high water pressure in the pipe.

(D) falls back down the pipe leading to the faucet and remains there until you reopen the faucet.

Problem 10:

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

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

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

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

Problem 11:

When you jump while standing on a bathroom scale, it briefly reads more than your actual weight. During that moment, it's exerting an upward force on you that is greater than your weight and

(A) you are accelerating upward.

(B) your velocity is constant but downward.

(D) you are accelerating downward.

Problem 12:

Like a baseball bat, a tennis racket has a sweet spot at its center of percussion. If a tennis ball hits this center of percussion, the racket's handle doesn't accelerate. That's because

(A) an impact at the center of percussion transfers no momentum to the racket and doesn't cause the racket to accelerate.

(B) the racket's velocity doesn't change when the ball hits its center of percussion.

(C) an impact at the center of percussion exerts no torque about the racket's center of mass and doesn't cause the racket to undergo angular acceleration.

(D) the racket's center of mass accelerates backward while its handle rotates forward and the two motions cancel one another at the handle.

Problem 13:

You can tell how much cargo a ship has on board by looking at how it floats. When the ship is full, it floats low in the water because the cargo increases the ship's

(A) momentum so that more water is needed to balance that momentum.

(B) moment of inertia so that the water needs more kinetic energy to keep the ship afloat.

(D) average density so that it must displace more water to stay afloat.

Problem 14:

You are riding on a swing at the local playground. As you swing back and forth, you begin to think about your speed and kinetic energy (this is obviously a fictional story). These two quantities clearly change between the top of each swing (when you are reversing directions) and the bottom of each swing (when you are passing directly beneath the supporting beam). You wonder when each of these two quantities is at its maximum value. Actually, your speed is at its maximum

(A) at the bottom of a swing and your kinetic energy is at its maximum at the bottom of a swing.

(B) at the bottom of a swing and your kinetic energy is at its maximum at the top of a swing.

(D) at the top of a swing and your kinetic energy is at its maximum at the top of a swing.

Problem 15:

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) Neither ball is pushing on the other.

(B) They are both pushing on one another.

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

Problem 16:

A book slides slowly off a tilted table. Its speed remains constant until it finally falls off the edge. While it's sliding down the table, the book doesn't accelerate because

(A) sliding friction is exerting an uphill force on it that exactly balances the downhill force due to gravity.

(B) angular momentum is a conserved quantity, so the book's angular momentum can't change.

(D) momentum is a conserved quantity, so the book's momentum can't change.

Problem 17:

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

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

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

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

Problem 18:

The total energy of a rubber ball in a box is contained in the ball's gravitational potential energy, its kinetic energy of motion, and its thermal energy. Energy can be transferred from one of these forms to another as the ball moves around. You throw the ball into the box and leave it for 10 minutes. When you return, most of the ball's energy will have

(A) turned into gravitational potential energy.

(B) turned into random bouncing of the ball around the box.

(D) turned into thermal energy.

Problem 19:

The pressure in an upright bottle of water, resting on a table, is

(A) zero, because water accelerates whenever it's subjected to pressure.

(B) the same throughout (not zero), because the water would accelerate if it were subjected to a pressure imbalance.

(D) highest near the bottom, because the water there must support the weight of the water above it.

Problem 20:

You are cleaning a wall by spraying water at it from a hose. At the center of the stream of water, right where it hits the wall, the water is coming to a complete stop. If you were to measure the water pressure at that point, you would find that it is

(A) equal to atmospheric pressure.

(B) lower than atmospheric pressure, but more than zero.

(D) exactly zero.

Problem 21:

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) your velocity is less as you land on the mattress than it would have been if you'd landed on the sand heap.

(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) 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.

Problem 22:

When you roll down the first big hill on a roller coaster, you feel particularly weightless because

(A) your mass is downward.

(B) your momentum is downward.

(D) your velocity is downward.

Problem 23:

You're trying to sleep on a train that is traveling forward at high speed. Suddenly you find yourself thrown toward the right. Your eyes are closed, so you can't see what happened but you know that the train has just

(A) turned toward the left.

(B) begun to ascend a hill.

(D) turned toward the right.

Problem 24:

You are floating along in a hot air balloon. You look up and notice that the bottom of the balloon is open. Hot air remains inside the balloon despite this opening because

(A) the air pressure inside the balloon's opening is the same as the air pressure outside that opening.

(B) hot air has more inertia than cold air and doesn't accelerate easily.

(D) the propane burner located below the opening keeps pushing the hot air back into the balloon.

Problem 25:

You're having trouble loosening a rusty nut with a small wrench, so you borrow a large wrench from your neighbor. Exerting only a modest force on the handle of this new wrench easily unscrews the nut. The large wrench helps because

(A) it has a large moment of inertia so that it develops a great deal of angular momentum when you exert a force on it.

(B) it has a large mass so that its inertia allows you to overcome the nut's velocity and accelerate it around in a circle.

(D) it has a large acceleration and a large mass, so the force it produces is large, according to the equation F=ma.

PART II: SHORT ANSWER QUESTIONS

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

Problem 1:

You're taking a step aerobics class. In front of you is a small platform that you step onto and off of during the course of the exercises. Most of the time, one foot remains stationary on the platform and you use it to lift your body up and down.

(A) As you step up onto the platform, your leg does work on your body. What characteristics of you and the platform determine how much work your leg must do?

(B) How much work is your leg doing on your body as it lowers you gently back down to the ground?

(C) If you let yourself drop back down to the ground, rather than lowering yourself gently, you may injure the leg you land on. Why does wearing padded shoes reduce your risk of injury?

(D) The platforms are all identical and are made of a sturdy plastic that acts like a stiff spring. When you step up onto yours, it distorts downward by about 4 millimeters. You're curious about the weight of the person to your right so you watch the platform as that person steps onto it. It distorts downward by 6 millimeters. How much does that person weigh?

Problem 2:

In the game of pool, the first shot or "break" is very important. The fifteen pool balls are arranged in a triangle and the player rolls another ball, the cue ball, into the triangle to separate the balls.

(A) The break starts when the player pushes the cue stick forward so that it strikes the stationary cue ball. This impact transfers energy to the cue ball. Show that the cue stick has done work on the cue ball.

(B) Just before it hits the stationary triangle of balls, the cue ball has 16 units of momentum in the forward direction. After the collision, the sixteen balls travel in all directions. If there were no friction or air resistance, what would be the total momentum of all the balls on the table just after the cue ball hit the triangle?

(C) Just before it hits the stationary triangle of balls, the cue ball has 1 joule of kinetic energy. After the collision, the balls have a total of only 0.7 joules of kinetic energy. What happened to the remaining 0.3 joules?

(D) When one pool ball collides directly with a second stationary ball, the first ball stops and the second ball continues the first ball's motion. The transfer of energy and momentum is almost perfect. This trick works because pool balls are extremely lively (they bounce well when dropped on the floor). If pool balls were perfectly dead (they didn't bounce at all when dropped on the floor), what would happen to the first ball after it collided with the second stationary ball?

Problem 3:

It's a hot summer day and you and your friends are having a water fight. You've visited the local toy store and brought home several fancy water guns and bags of water balloons.

(A) The water guns store energy in compressed air. As you push a handle back and forth, a pump squeezes more and more air molecules into a plastic container that's partly filled with water. Why does the air pressure inside this container increase as you pack the molecules into it?

(B) The compressed air pressurizes the water, which flows slowly through a tube and then squirts out a narrow nozzle at high speed. How does the water's pressure change as it flows out this nozzle?

(C) One of your friends begins to squirt water down at you from the second floor balcony. You begin to squirt water up at her but it comes to a stop before it reaches her. You pump some more air into the container and then you're able to squirt her. Why did increasing the water pressure help?

(D) Another friend begins to lob water balloons at you. His aim is poor and one of the balloons hits the window behind you and breaks it. Water can only use pressure to push on a surface and the pressure in the water balloon was roughly atmospheric pressure while it was flying through the air. So how was the balloon able to push the window hard enough to break it?