Given Friday, October 11, from 1:00 PM to 1:50 PM

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:

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) rocking away from you.

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

(C) rocking toward you.

(D) on your side of its equilibrium position.

Answer: (A) rocking away from you.

Problem 2:

After losing a tightly contested Tiddly-Winks tournament at UVA, several of the visiting players are driving recklessly down Emmet street at 120 mph. The police car chasing them is also traveling 120 mph. To avoid hitting a scout, who is walking an elderly person across the street to U-Hall, the visiting players slow their car to 110 mph. The police car overtakes the players' car and collides with its rear end. If both cars were traveling at constant velocities before the collision, the police car will

(A) accelerate forward during the collision because of its inertia.

(B) accelerate backward during the collision.

(C) not accelerate during the collision.

(D) accelerate forward during the collision because of its forward velocity.

Answer: (B) accelerate backward during the collision.

Problem 3:

You're playing darts at the Greenskeeper on the corner. You find that the slower you throw a dart, the higher you have to aim in order to hit the center of the dart board. That's because the slow moving dart

(A) has more centrifugal force than a faster moving dart.

(B) weighs more than a faster moving dart.

(C) has more time to fall during its longer flight than a faster moving dart would have.

(D) has more mass than a faster moving dart.

Answer: (C) has more time to fall during its longer flight than a faster moving dart would have.

Problem 4:

You are determined to enter the hammer-throw competition in the next Olympics. To practice, you have tied a Bodo's bagel to the end of a string and are swinging it furiously in a circle directly over your head. To a friend looking down on you from the balcony above, the bagel is traveling in a counter-clockwise circle around your head. If you want the bagel to travel directly forward, you should let go of the string at a moment when the bagel is located

(A) directly in front of you.

(B) almost directly in front of you, but still somewhat to your right.

(C) directly to your right.

(D) almost directly to your right, but already somewhat in front of you as well.

Answer: (C) directly to your right.

Problem 5:

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

(D) 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.

Answer: (D) 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.

Problem 6:

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) lower than atmospheric pressure, but more than zero.

(B) equal to atmospheric pressure.

(C) higher than atmospheric pressure.

(D) exactly zero.

Answer: (C) higher than atmospheric pressure.

Problem 7:

You're playing intramural volleyball and the other team has hit the ball directly to you. You jump up and hit the ball back across the net with just the right speed to score a point. If the ball had been coming toward you faster, how would that have affected the ball's speed after you hit it, assuming that you still hit the ball the same way?

(A) The faster-moving ball would travel away from you more slowly after you hit it because of its centrifugal force.

(B) The faster-moving ball would also travel away from you faster after you hit it.

(C) The faster-moving ball would travel away from you more slowly after you hit it because of the force of its momentum.

(D) The faster-moving ball would travel away from you more slowly after you hit it because it would need more acceleration to reverse its velocity.

Answer: (B) The faster-moving ball would also travel away from you faster after you hit it.

Problem 8:

You stop for a cappuccino at Greenberry's and notice that the tiny white bubbles of steamed milk remain on the surface of the coffee. These air-filled bubbles stay where they are, rather than descending into the coffee or rising into the air, because they are

(A) less dense than the coffee but more dense than the air above the coffee.

(B) more dense than the coffee but less dense than the air above the coffee.

(C) lighter than the cup of coffee but heavier than the column of air above the coffee.

(D) more viscous than the coffee but less viscous than the air above the coffee.

Answer: (A) less dense than the coffee but more dense than the air above the coffee.

Problem 9:

You're riding on a pogo stick-a vertical stick with a spring at the bottom and foot pads on which you stand. You're holding the top of the stick tightly in your hands and you and the stick are bouncing up and down on its spring. As you land after one particularly high bounce, the spring is becoming more and more tightly compressed. Just before the spring reaches its tightest compression, your velocity is

(A) downward but your acceleration is upward.

(B) downward and you acceleration is downward.

(C) upward and your acceleration is upward.

(D) upward but your acceleration is downward.

Answer: (A) downward but your acceleration is upward.

Problem 10:

Two hikers are carrying identical backpacks. They start in the parking lot at the base of a 1,500 meter mountain and hike to the top of that mountain. The first hiker climbs directly up the steep slope to the top, while the second hiker walks along the gradually sloping road that circles the mountain several times before reaching the top. The first hiker arrives at the top first. If both backpacks were motionless at the begin and end of the hike, which hiker did the most work on her backpack?

(A) Neither. They did equal amounts of work on their backpacks.

(B) The second hiker, because she walked the greater distance with her backpack.

(C) The first hiker, because she exerted the larger uphill force on her backpack.

(D) The first hiker, because she took less time to carry the backpack up the mountain.

Answer: (A) Neither. They did equal amounts of work on their backpacks.

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 downward.

(B) your velocity is constant but downward.

(C) your velocity is constant but upward.

(D) you are accelerating upward.

Answer: (D) you are accelerating upward.

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 exerts no torque about the racket's center of mass and doesn't cause the racket to undergo angular acceleration.

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

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

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

Answer: (B) 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're filling a jar of honey from the spigot at the bottom of a large barrel at the grocery store. The honey flows extremely slowly, so the store manager has the barrel refilled. Now the honey flows much more rapidly from the spigot because

(A) the pressure of the honey at the bottom of the barrel increases as the height of honey in the barrel increases.

(B) the density of the honey decreases as the height of the honey in the barrel increases.

(C) the viscosity of the honey (its "thickness") decreases as the height of honey in the barrel increases.

(D) the viscosity of the honey (its "thickness") increases as the height of honey in the barrel increases.

Answer: (A) the pressure of the honey at the bottom of the barrel increases as the height of honey in the barrel increases.

Problem 14:

The curves on bicycle racetracks are steeply banked, so that the inner edge of each curve is much lower than its outer edge. This banking tips the support force that the track exerts on the bicycle wheel toward the center of each turn. That center-directed or centripetal force on the bicycle is important because it

(A) helps the bicycle accelerate inward to complete each turn without skidding.

(B) it does work on the bicycle during each turn and thus increases the bicycle's energy.

(C) balances the outward centrifugal force that the bicycle experiences as it completes each turn, so that the bicycle experiences zero net force.

(D) it does negative work on the bicycle during each turn and thus decreases the bicycle's energy.

Answer: (A) helps the bicycle accelerate inward to complete each turn without skidding.

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) 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 in front is pushing backward on the ball behind.

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

(D) Neither ball is pushing on the other.

Answer: (D) Neither ball is pushing on the other.

Problem 16:

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

(A) hot air has a lower pressure than cold air, so hot air is drawn into the balloon by the partial vacuum inside it.

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

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

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

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

Problem 17:

You are playing ice hockey at the skating rink and someone has just flicked the puck toward you. It's zipping across the ice at very high speed and you want to stop it with the flat surface at the end of your hockey stick. You are aware that the puck is an elastic object that bounces well. To slow the puck down most effectively as it hits your stick, you should

(A) move your stick aggressively toward the oncoming puck.

(B) keep your stick as stationary as possible.

(C) move your stick in the direction that the puck is moving.

(D) move your stick gently toward the oncoming puck.

Answer: (C) move your stick in the direction that the puck is moving.

Problem 18:

You hold a new glass bottle of root beer upright in one hand and pound its bottle cap downward hard with a rubber mallet. As a consequence of this action,

(A) the sudden acceleration will cause the liquid in the bottle to float above the gas in the bottle for several seconds.

(B) the surge in pressure near the top of the bottle will cause the bottle cap to vibrate loudly.

(C) liquid colliding with the bottom of the bottle will knock the bottom out of the bottle.

(D) the sudden expansion of gas inside the bottle will cause the top layer of root beer to freeze.

Answer: (C) liquid colliding with the bottom of the bottle will knock the bottom out of the bottle.

Problem 19:

A rubber balloon that's filled with helium gas floats in air while an identical balloon that's filled with air sinks in air. That's because,

(A) while helium atoms and air molecules have equal masses, helium atoms weigh less than air molecules.

(B) while helium atoms and air molecules have equal weights, the helium balloon contains fewer particles than the air-filled balloon.

(C) while helium atoms and air molecules have equal weights, helium atoms have less mass than air molecules.

(D) while both balloons contain the same number of particles, helium atoms are much lighter than air molecules.

Answer: (D) while both balloons contain the same number of particles, helium atoms are much lighter than air molecules.

Problem 20:

When a Frisbee is flying horizontally through the air, the speed of the air flowing over the Frisbee's top is

(A) higher than the speed of the air beneath it and the pressure on top of the Frisbee is greater than the pressure beneath it.

(B) lower than the speed of the air beneath it and the pressure on top of the Frisbee is less than the pressure beneath it.

(C) lower than the speed of the air beneath it and the pressure on top of the Frisbee is greater than the pressure beneath it.

(D) higher than the speed of the air beneath it and the pressure on top of the Frisbee is less than the pressure beneath it.

Answer: (D) higher than the speed of the air beneath it and the pressure on top of the Frisbee is less than the pressure beneath it.

Problem 21:

A curve ball's path bends to the right as it flies toward home plate because it's experiencing an aerodynamic force to its right. One reason why the ball is experiencing this aerodynamic lift force is that the ball is spinning and that spin

(A) causes the air to flow more rapidly around the left side of the ball than around the right side of the ball.

(B) exerts a torque on the ball that causes its angular momentum to shift toward the left.

(C) exerts a torque on the ball that causes its angular momentum to shift toward the right.

(D) causes the air to flow more rapidly around the right side of the ball than around the left side of the ball.

Answer: (D) causes the air to flow more rapidly around the right side of the ball than around the left side of the ball.

Problem 22:

In an attempt to rob the stagecoach, the bandits pull out a nail gun that they stole from a construction site and nail the coach's wheels firmly to its frame. The wheels can no longer turn and the coach abruptly skids to a stop. What has become of the coach's kinetic energy?

(A) It's now elastic potential energy in the nails.

(B) It's now thermal energy in the wheels and ground.

(C) It's now gravitational potential energy in the wheels.

(D) It's still present in the coach, as it must be because kinetic energy is conserved and can't be created or destroyed.

Answer: (B) It's now thermal energy in the wheels and ground.

Problem 23:

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) only the downward force of its weight, both before and after it reaches its maximum height.

(B) 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.

(C) 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.

(D) 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.

Answer: (A) only the downward force of its weight, both before and after it reaches its maximum height.

Problem 24:

As air flows through the horizontal ductwork in a warehouse, that air passes through a fan inside the duct and the air's total energy increases. Since the air's speed doesn't change as the result of going through the fan, you know that the air's

(A) kinetic energy has decreased but its pressure has remained the same.

(B) kinetic energy has increased but its pressure has remained the same.

(C) pressure has decreased but its kinetic energy has remained the same.

(D) pressure has increased but its kinetic energy has remained the same.

Answer: (D) pressure has increased but its kinetic energy has remained the same.

Problem 25:

You're seated in the dining car of a train, facing forward so that you can watch the scenery come toward you. As you cut your roll, the apple that's been sitting motionless on the level table suddenly rolls to your right. You know that the train

(A) is moving at constant velocity toward the right.

(B) has accelerated toward the left.

(C) is moving at constant velocity toward the left.

(D) has accelerated toward the right.

Answer: (B) has accelerated toward the left.

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 have fashioned a modest catapult out of a truck's leaf spring-a stiff, elastic strip of springy metal that's shaped like a very large ruler. One end of the strip is bolted to wagon full of sand and the other end of the strip is carved into a bowl. You put an apple in the bowl, bend the strip backward with your foot, and then step out of the way. The apple flies several hundred feet into the air.

(A) When you step on the bowl end of the metal strip, it bends downward 50 centimeters. If someone half your weight were to step on the strip, how far downward would it bend?

Answer: 25 centimeters.

(B) You're holding the bowl-end of the strip down with your foot and the apple in the bowl isn't moving. What forces are acting on the apple?

Answer: Its weight (downward) and a support force (upward).

(C) You step off the bent strip and it quickly straightens out, flinging the apple skyward. As the strip straightens out and pushes the apple upward, which force is larger: the force the apple exerts on the strip or the force the strip exerts on the apple?

Answer: The forces are equal (in magnitude).

(D) As the apple rises, its upward momentum gradually decreases. Neglecting air resistance, what is the apple transferring its momentum to?

Answer: To the earth (by way of gravitational force).

Problem 2:

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?

Answer: Your weight and the height of the platform.

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

Answer: The product of your weight times the height of the platform.

(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?

Answer: It slows your descent over a longer time with a smaller force.

(D) The platforms are all identical and are made of a sturdy plastic that acts like a stiff spring. When you step up onto your platform, 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?

Answer: 1.5 times your weight.

Problem 3:

At the start of a kayak race, you're sitting motionless in your kayak and the kayak is motionless on the water.

(A) How much force is the water exerting on the kayak (give both the amount of that force and its direction).

Answer: The force is upward and equal in amount (or magnitude) to the total weight of the kayak and you.

(B) The starting gun goes off and you pull your paddle rapidly through the water, from the front of the boat to the rear. Your kayak accelerates forward. What force is causing the kayak to accelerate forward?

Answer: The water is exerting a forward drag force on the paddle.

(C) When you pull the paddle rapidly through the water, it leaves a string of whirlpools behind it. These whirlpools wouldn't form if you were to pull the paddle very slowly through the water. Why not?

Answer: At low speeds (and low Reynold's numbers), the water flow around the paddle would be laminar.

(D) You watch water in the stream flow smoothly against a large rock and notice that the water level rises just in front of the rock. What is providing the energy needed to lift the water upward and what is happening to the speed of the water as it encounters the rock?

Answer: The water's kinetic energy provides the energy and the water slows down as it encounters the rock.