Physics 105 - How Things Work - Fall, 2002

Midterm Examination - Solutions

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:

A springboard diver leaps upward from the springboard, rises dramatically to a peak height, and than drops impressively into the water below the board. Neglect any influences of air or the atmosphere. During this trip, the diver experiences

(A) an upward net force that gradually diminishes to zero at the peak height and then becomes a downward net force.
(B) a constant upward net force on the way up and a constant downward net force on the way down.
(C) a constant downward net force.
(D) a downward net force that is proportional to the diver's height above the water.
Answer: (C) a constant downward net force.
Why: The only force acting on the free-falling diver is the diver's weight. The diver accelerates downward at the acceleration due to gravity.

Problem 2:

You are part of a team playing tug-of-war with another team. Each team is pulling on one end of a rope and trying to drag the other team into a patch of mud in the middle of the horizontal playing field. Your team is lighter than the other team, but the two teams are about equally strong and no one is making any progress. While the two teams are essentially motionless, the frictional force the other team experiences from the ground is

(A) less than but opposite to the frictional force your team experiences from the ground.
(B) not directly related to the frictional force your team experiences from the ground.
(C) greater than but opposite to the frictional force your team experiences from the ground.
(D) equal but opposite to the frictional force your team experiences from the ground.
Answer: (D) equal but opposite to the frictional force your team experiences from the ground.
Why: The two teams are not accelerating, so the net force on them as a pair is zero. Thus, the frictional force on one team must balance the frictional force on the other team.

Problem 3:

A two-inch fire hose can carry over 200 times as much water per second as a half-inch garden hose, even though both hoses have the same length and are fed with water at the same pressure. The basic reason for this difference is that the fire hose

(A) exerts a much larger forward force on the water passing through it and makes that water accelerate much more rapidly.
(B) has a much larger forward momentum to pass along to the water inside it.
(C) holds much more water and gets most of that water far away from the stationary walls of the hose.
(D) has much more energy in its walls than the garden hose and conveys some of that energy to the water passing through it.
Answer: (C) holds much more water and gets most of that water far away from the stationary walls of the hose.
Why: The walls of the hose slow the nearby water, so getting the main flow of water far away from those walls greatly increases the water's speed and the hose's capacity.

Problem 4:

You are trying to knock over a heavy bottle by hitting it with a 100-gram object and transferring momentum to it. You have two such objects: an almost perfectly elastic "bouncy ball" and an almost perfectly dead beanbag. If you throw the two objects equally fast, then the bouncy ball will transfer

(A) a large amount of momentum to the bottle, while the beanbag will transfer almost none.
(B) about twice as much momentum to the bottle as the beanbag will.
(C) about half as much momentum to the bottle as the beanbag will.
(D) about the same amount of momentum to the bottle as the beanbag will.
Answer: (B) about twice as much momentum to the bottle as the beanbag will.
Why: While both objects initially transfer all of their forward momentum to the bottle, only the bouncy ball does more. The bouncy ball kicks back and transfers still more forward momentum to the bottle. By rebounding at its initial speed, but in the opposite direction, the bouncy ball manages to completely reverse its own momentum and therefore transfer twice as much forward momentum to the bottle.

Problem 5:

You've missed a turn on a go-cart track and you're headed for disaster. You can crash into either a brick wall or a haystack. After a rapid review of all the physics you know, you decided to aim for the haystack. Good choice. Although either obstacle would have brought you to a complete stop, the haystack extracted

(A) less momentum from you than the brick wall would have.
(B) the same amount of momentum from you that the brick wall would have, but while exerting less force on you.
(C) almost no momentum from you and thereby exerted less force on you than the brick wall would have.
(D) more momentum from you than the brick wall would have.
Answer: (B) the same amount of momentum from you that the brick wall would have, but while exerting less force on you.
Why: You will lose all of your momentum as you come to a stop, but the haystack extracts that momentum more slowly than the wall and thus exerts smaller forces on you to stop you.

Problem 6:

A steady, horizontal stream of water from a hose strikes a wall. As the water hits the wall, its velocity is forward, its acceleration is

(A) backward, and its pressure is increasing.
(B) forward, and its pressure is increasing.
(C) backward, and its pressure is decreasing.
(D) forward, and its pressure is decreasing.
Answer: (A) backward, and its pressure is increasing.
Why: The water slows as it approaches the wall (since it can't go through the wall). That slowing requires a backward acceleration and must be caused by a higher pressure in front of the water (near the wall's surface) than in the approaching stream.

Problem 7:

When you first step onto a bathroom spring scale, the value it reports increases rapidly to more than your actual weight. It takes a second or two before the scale gives an accurate reading of your weight. The reason the scale reads too high at first is that

(A) you coast downward through the equilibrium height and do not stop there.
(B) your acceleration is always downward at 9.8 meters per second squared.
(C) your mass and your weight are not equal at first and the scale takes several seconds to balance them.
(D) your acceleration is always upward while you are touching the scale's surface, but that acceleration is too weak to stop your descent at first.
Answer: (A) you coast downward through the equilibrium height and do not stop there.
Why: Although you eventually settle at the equilibrium point, your downward momentum initially carries you through it at a constant downward velocity. An equilibrium point is not a stopping point, but rather a coasting point.

Problem 8:

You are trying to loosen an ordinary screw that someone has jammed into your dormitory wall. Try as you may, you can't get that screw to turn at all. While you are twisting as hard as possible on the head of the screw, the net torque that the screw is experiencing is

(A) either clockwise or counterclockwise, depending on whether you are in the northern or southern hemisphere.
(B) counterclockwise.
(C) zero.
(D) clockwise.
Answer: (C) zero.
Why: Since the screw is remaining motionless, it is experiencing zero angular acceleration and thus zero net torque.

Problem 9:

When a spinning skater pulls his arms in close to him, his rate of rotation increases dramatically. During this process, his moment of inertia

(A) stays constant, his angular velocity increases, and his angular momentum decreases.
(B) decreases, his angular velocity increases, and his angular momentum stays constant.
(C) increases, his angular velocity decreases, and his angular momentum stays constant.
(D) stays constant, his angular velocity decreases, and his angular momentum increases.
Answer: (B) decreases, his angular velocity increases, and his angular momentum stays constant.
Why: By isolating himself on the tip of his skate, the skater manages to prevent torques from altering his angular momentum. His angular momentum therefore remains constant. Since that angular momentum is composed of the product of his moment of inertia times his angular velocity, when he shrinks his moment of interia by pulling his arms in, he makes it essentially that his angular velocity increase to compensate.

Problem 10:

You can bicycle up a particular hill in one of several gears, making the bicycle easier or harder to pedal. In going from rest at the bottom of the hill to rest at the top of the hill, which of the following quantities will be the same regardless of which gear you use?

(A) The momentum you transfer to the pedals times the distance the pedals move in the direction of that momentum.
(B) The momentum you transfer to the pedals times the time during which you are pushing on those pedals.
(C) The force you exert on the pedals times the distance the pedals move in the direction of that force.
(D) The force you exert on the pedals times the time during which you are pushing on those pedals.
Answer: (C) The force you exert on the pedals times the distance the pedals move in the direction of that force.
Why: Raising the bicycle to the top of that hill will require a certain amount of work, no matter how you do it. Since you are the source of that work, the force you exert (on the pedals) times the distance (the pedals) travelled is always the same, regardless of gearing.

Problem 11:

If you swing a full bucket of water over your head quickly enough, the water will stay in the bucket even when that bucket is upside-down. If you swing the bucket more slowly, you'll get wet. One way to understand these different behaviors is to note that

(A) inertia dominates the water's motion at high speeds, while gravity dominates at low speeds.
(B) gravity dominates the water's motion at high speeds, while inertia dominates at low speeds.
(C) weight dominates the water's motion at high speeds, while gravity dominates at low speeds.
(D) gravity dominates the water's motion at high speeds, while weight dominates at low speeds.
Answer: (A) inertia dominates the water's motion at high speeds, while gravity dominates at low speeds.
Why: At high speeds, the entire swing is over so quickly that gravity has essentially no time to affect the water's motion. In that case, the water's inertia places the key role in its motion and succeeds in keeping the water in the bucket. But at low speeds, the swing is so slow that gravity has plenty of time to affect the water and its motion. You get wet.

Problem 12:

Scuffing the soles of your shoes on the sidewalk wears them out. You do much better to plant each sole firmly against the pavement, move your body forward, and then lift that sole cleanly back off the pavement. Keeping each sole in place as it touches the sidewalk dramatically reduces its wear because then the sole doesn't

(A) experience any friction at all.
(B) experience any sliding friction.
(C) do any work on the sidewalk.
(D) transfer any momentum to the sidewalk.
Answer: (B) experience any sliding friction.
Why: Static friction is essential to walking, particularly to starting and stopping. But sliding friction is not so helpful and both wastes energy and damages your shoes. By planting your feet and not sliding them, you obtain the horizontal forces of static friction that you need to accelerate horizontally, but you don't do any work against static friction and don't wear out your shoes.

Problem 13:

The drought has dried up your favorite water skiing lake, so you have no choice but to ski on the highway behind a car. It's fun, but it sure isn't good for your skies. A rope attached to the car is pulling you straight ahead along a level, horizontal road at a constant speed of 60 miles per hour. The net force on you is

(A) pointing in the upward direction.
(B) pointing in the backward direction.
(C) zero.
(D) pointing in the forward direction.
Answer: (C) zero.
Why: Despite all the apparent complexity of this situation, you are traveling at constant velocity and therefore are not accelerating. The net force on you must be zero.

Problem 14:

Consider two automobile accidents in which identical cars are driven into a solid wall at 30 mph and 60 mph, respectively. Compared to the car traveling 30 mph, the car 60 mph transfers

(A) four times as much momentum into the wall and converts twice as much kinetic energy into other forms.
(B) twice as much momentum into the wall and converts twice as much kinetic energy into other forms.
(C) the same momentum into the wall and converts twice as much kinetic energy into other forms.
(D) twice as much momentum into the wall and converts four times as much kinetic energy into other forms.
Answer: (D) twice as much momentum into the wall and converts four times as much kinetic energy into other forms.
Why: Momentum is proportional to speed, but kinetic energy is proportional to speed squared. When you double a car's speed, you only double its momentum but you quadruple its kinetic energy.

Problem 15:

When you jump on a trampoline, you stretch an elastic surface up and down. The two of you (you and the trampoline) have a combined total potential energy that changes as you bounce around. The point at which your combined total potential energy is lowest is when you are located

(A) at the lowest point in your bounce, when you have dropped as low and deep into the trampoline's surface as possible.
(B) just above the trampoline's surface, while that surface is flat, unstretched, and horizontal.
(C) at the highest point in your bounce above the trampoline.
(D) at the equilibrium height to which you settle down if you stop jumping.
Answer: (D) at the equilibrium height to which you settle down if you stop jumping.
Why: The equilibrium height is a stable equilibrium, meaning that you accelerate toward it whenever you are above or below. Since you always accelerate in the direction that lowers your total potential energy as quickly as possible, that stable equilibrium must be lower in total potential energy than anywhere above or below that equilibrium. The equilibrium is therefore the point of lowest total potential energy.

Problem 16:

You drop a ball from rest on a concrete floor and let it bounce twice. The first time it rises to 80% of its original height before descending again. The second time it rises to

(A) 64% of its original height.
(B) 60% of its original height.
(C) 20% of its original height.
(D) 40% of its original height.
Answer: (A) 64% of its original height.
Why: From the first bounce, we know that the ball retains 80% of its collision energy following any bounce. Following the second bounce, it should 80% of the 80% it had after the first bounce, or 64% of its original energy. Since height and gravitational potential energy change in direct proportion, when the ball has only 64% of its original energy, it can rise to only 64% of its original height.

Problem 17:

Airports often provide you with moving sidewalks to help you get from one place to another quickly and easily. Suppose you are riding one of these sidewalks up a gentle hill to your departure gate. You are traveling at a constant velocity, forward and slightly upward. Neglect any air resistance. During this time, the sidewalk is pushing you

(A) upward and forward, but it is not doing any work on you.
(B) in the direction you are moving (up the hill) and it is doing work on you.
(C) straight up and it is doing work on you.
(D) in the direction you are moving (up the hill) and you are doing work on the sidewalk.
Answer: (C) straight up and it is doing work on you.
Why: You are traveling at constant velocity, so the net force on you must be zero. Since the sidewalk must be balancing your downward weight, it must be pushing directly upward on you with a force equal in amount to your weight. Because it pushes upward on you and you move at least partially in the upward direction, the sidewalk is also doing work on you.

Problem 18:

You find that when you load up the door to your refrigerator with bottles of soda, the door becomes relatively hard to open or shut. It doesn't respond quickly to your pushes or pulls. To minimize this effect, you should put the soda bottles as close as possible to the

(A) bottom of the door.
(B) outside edge of the door (away from the hinges).
(C) inside edge of the door (near the hinges).
(D) top of the door.
Answer: (C) inside edge of the door (near the hinges).
Why: The difficulty in opening or shutting the door is the result of a large moment of inertia: the door has substantial rotational inertia. To minimize this rotational inertia, place the mass as close as possible to the axis of rotation. By putting the bottles near the hinges, you minimize their contribution to that moment of inertia.

Problem 19:

Which of the following controls can cause an automobile to accelerate: the accelerator pedal, the brake pedal, and the steering wheel.

(A) only the accelerator pedal and the steering wheel.
(B) all three.
(C) only the accelerator pedal and the brake pedal.
(D) only the accelerator pedal.
Answer: (B) all three.
Why: An automobile accelerates whenever it changes speed or direction or both. All three controls can do this; the accelerator and brake directly affect speed, while the streering wheel affects direction.

Problem 20:

A unicycle has only a single wheel and its rider sits above that wheel, pedaling furiously to stay upright. When she is traveling straight forward at a steady pace, she can maintain stability by always placing her overall center of gravity

(A) directly above the point at which the wheel touches the ground.
(B) slightly in front of the point at which the wheel touches the ground.
(C) slightly in front of her center of mass.
(D) slightly behind the point at which the wheel touches the ground.
Answer: (A) directly above the point at which the wheel touches the ground.
Why: When she is traveling at constant velocity, she is experience no horizontal force from the ground and therefore no frictional torques on her. She must therefore be careful to avoid any torques due to the support force that the ground is exerting on her wheel. By placing her center mass/gravity directly above her wheel's contact point with the ground, she gets rid of any torque that the ground would otherwise exert on her and she reaches equilibrium. Although that equilibrium is unstable, she can maintain it with fancy footwork and careful balance.

Problem 21:

You shoot an arrow straight up, using a bow. Naturally, you run away before it comes back down. While the arrow is heading upward, but no longer touching the bow, the net force on it is

(A) upward and it is accelerating upward.
(B) downward and it is accelerating upward.
(C) upward and it is accelerating downward.
(D) downward and it is accelerating downward.
Answer: (D) downward and it is accelerating downward.
Why: The arrow is a freely falling object. Although it is heading upward, the only force acting on it is its downward weight and it is accelerating downward.

Problem 22:

You have three identical one-liter containers. One is filled with solid lead, the second is filled with liquid water, and the third is filled with gaseous helium. You push all three containers to the bottom of a tank of water and hold them there. Which container experiences the largest upward buoyant force?

(A) The container filled with water.
(B) All three containers experience the same buoyant force.
(C) The container filled with lead.
(D) The container filled with helium.
Answer: (B) All three containers experience the same buoyant force.
Why: The buoyant force on an object depends only on its volume and on the fluid in which it is immersed, not on what the object contains.

Problem 23:

You're seated in the last car of a roller coaster train and being pulled over the lip of the first plunging drop. As you go over this lip, you feel nearly weightless. This experience of weightlessness is more severe for you than it was for people in the first car of the train because you

(A) are accelerating downward at a greater rate than the people in the first car.
(B) have a greater speed than the people in the first car.
(C) have a greater velocity than the people in the first car.
(D) are accelerating upward at a greater rate than the people in the first car.
Answer: (A) are accelerating downward at a greater rate than the people in the first car.
Why: The weightless feeling is a direct result of downward acceleration. Since you are whipped over that first lip, you accelerate downward particularly quickly and feel an intense weightlessness.

Problem 24:

Through a strange mishap during a flight to Miami Beach, a scuba tank full of extremely high pressure air falls out of an airplane. It has been falling for a while and is now just 10 feet above the ground. The air inside the tank is moving much faster than the air outside the tank and it also has much greater pressure. Why doesn't this arrangement violate Bernoulli's equation?

(A) The air outside the tank has enough weight to compensate for the differences in pressure and speed.
(B) The air outside the tank has enough gravitational potential energy to compensate for the differences in pressure and speed.
(C) The air inside and the air outside the tank have different histories and do not necessarily have the same total energies per quantity.
(D) The air outside the tank has enough kinetic energy to compensate for the difference in pressure and speed.
Answer: (C) The air inside and the air outside the tank have different histories and do not necessarily have the same total energies per quantity.
Why: Bernoulli's equation is wonderfully useful when it applies. However, it only applies to steady-state flows and even then, only along a single streamline. The fluid you study using Bernoulli's equation thus has to have nothing that can do work on it and it must have a consistent history. The air in the tank has no common history with the air around it and can't be compared using Bernoulli's equation in any meaningful way.

Problem 25:

While some fires can be fought with hoses connected directly to fire hydrants, other fires require that the water first pass through the pumps inside a fire truck. Adding energy to the water with a pump allows that water to

(A) travel to greater heights, but not reach greater pressures or achieve greater speeds.
(B) travel to greater heights, reach greater pressures, and achieve greater speeds.
(C) achieve greater speeds, but not reach greater pressures or travel to greater heights.
(D) achieve greater speeds and reach greater pressures, but not travel to greater heights.
Answer: (B) travel to greater heights, reach greater pressures, and achieve greater speeds.
Why: The pump adds energy to the water and that added energy can eventually be manifest in any of the energy forms: gravitation potential, pressure potential, and/or kinetic.

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:

What could be more fun than designing outrageous stunts for a comedy western movie? With your vast understanding of physics, you're earning the big bucks as one of Hollywood's best. You are going to end your current film by having the heroine save the hero from a hanging. As this scene begins, the hero is standing still on a trapdoor high above the ground and there is a rope around his neck. Suddenly the trapdoor opens and the hero has nothing beneath his feet.

(A) The rope is limp at first. The hero is experiencing what (1) net force, (2) velocity, and (3) acceleration? (report specific values for these three quantities)

Answer: The net force on him is his weight, his velocity is initially zero, and his acceleration is downward at the acceleration due to gravity (or, equivalently, 9.8 meters per second squared).
Why: Although he has not yet begun to descend and hasn't had time to acquire a downward velocity greater than zero, he is pulled down by gravity and is accelerating downward as a freely falling object.

(B) The heroine has secretly replaced the stiff rope with a stretchy elastic cord. Instead of pulling tight suddenly, the cord tenses gradually as the hero descends. At what point in this descent does the hero reach his maximum downward velocity?

Answer: When he reaches (or passes through) the equilibrium.
Why: At equilibrium, the cord pulls upward on him with a force that exactly balances his downward weight. He then experiences a net force of zero and is not accelerating. As he descends toward equilibrium, he accelerates downward (toward the stable equilibrium) and goes faster and faster. After he passes through equilibrium on his way down, he accelerates upward (toward the stable equilibrium) and goes slower and slower. His top speed occurs right at the moment of passing through equilibrium.

(C) The heroine gallops up on a white horse, passing right under the descending hero. He slows to a stop just as she reaches him and she cuts the cord from his neck. At the moment he stopped descending (and she cut him free), was the hero accelerating vertically and, if so, which way?

Answer: Yes, he was accelerating upward.
Why: He stops descending when his downward velocity is reduced to zero by an upward net force. He is well below the equilibrium height and the cord is pulling upward on him harder than his weight is pulling him downward. He reaches his lowest height and comes to a stop, but he is at that moment accelerating upward as rapidly as ever occurs during this scenario.

(D) The elastic cord snaps upward violently after being cut and it knocks the evil judge off the platform. Clearly, the cord has enormous kinetic energy at the end of the scene. What form was that energy in at the beginning of the scene?

Answer: Gravitational potential energy (in the hero)
Why: The cord received its energy when the hero stretched it and did work on it. The energy that allowed the hero to stretch the cord originated as gravitational potential energy in the hero. As he descended, his gravitational potential decreased and became both kinetic energy in him and elastic potential energy in the cord.

Problem 2:

You are playing soccer with your friends.

(A) As you kick the ball and it accelerates forward, how does the force you exert on the ball compare to the force the ball exerts on your foot?

Answer: The forces are equal in amount, but opposite in direction.
Why: These two forces, you pushing on the ball and the ball pushing on you, are a Newton's third law pair. They must be equal but opposite, no matter what is happening to you and the ball.

(B) While the ball is in the air after you kick it, which of the following physical quantities remains constant? (1) the ball's momentum, (2) the ball's angular momentum about its center of mass, (3) the ball's velocity, (4) the ball's acceleration, (5) the ball's total energy. Neglecting air resistance.

Answer: (Constant are:) the ball's angular momentum, its acceleration, and its total energy.
Why: In free fall, the ball experiences a downward force of gravity that changes both its velocity and its momentum. Neither of those two quantities is constant. However, gravity exerts no torque on the ball about its center of mass, so the ball's angular momentum about its center of mass is constant. The falling ball is accelerating downward steadily at the acceleration due to gravity, so that acceleration is constant. And it does no work on anything, so its total energy is constant.

(C) Still neglecting air resistance, describe the horizontal forces on the airborne ball during the period after it leaves your foot until just before it bounces on the ground 50 feet away.

Answer: There are no horizontal forces on the ball.
Why: The ball coasts forward because of its inertia. It experiences no horizontal pushes at all (neglecting air resistance).

(D) You kick the ball twice. The first time, the ball was stationary when you kicked it and the second time, the ball was moving toward you quickly when you kicked it. The motion of your leg was identical in each case. Which time did you kick the ball farther? Explain BRIEFLY.

Answer: You kick the approaching ball farther. It bounces harder off your leg and rebounds faster.
Why: From your leg's (moving) perspective, the stationary ball approaches your leg slowly and doesn't bounce very hard or fast. It ends up moving away from your leg, but not all that quickly. On the other hand, the ball that started by moving toward your leg has an advantage. From your leg's perspective, that ball approaches your leg rapidly and bounces hard and fast. It ends up moving away from your leg much more quickly and has a large outgoing velocity in the field's frame of reference.

Problem 3:

You are riding in a large helium-filled balloon that is passing over a city park. Down below, there is a huge picnic going on and you are watching the festivities.

(A) You are maintaining a constant altitude of about 100 feet above the ground. Compare the total weight of the balloon and its contents to the weight of the air that these objects are displacing. BRIEFLY justify your answer.

Answer: The two weights are equal. Since the objects don't accelerate, the buoyant force on the objects must equal their weight and that buoyant force is equal to the weight of the displaced air.
Why: The balloon is maintaining not only a constant height, but also a constant acceleration of zero. The upward buoyant force on it must thus balance its downward weight. Since the buoyant force on the balloon is equal to the weight of the air it displaces, the weight of the balloon and the weight of the air it displaces must be equal.

(B) You use a pump and a rigid gas storage tank to control the vertical motion of the balloon. To make your balloon descend, you pump helium out of the balloon and into the tank. To make your balloon rise, you let helium flow out of the tank and into the balloon. Why does this scheme work?

Answer: Pumping helium into the tank shrinks the balloon without changing its mass, so its average density increases and it sinks. Releasing helium decreases the average density so it rises.
Why: The helium balloon floats when its average density equals that of air. By packing the balloon more tightly (same mass in a smaller volume), the pumping system raises the balloon's average density and allows it to descend. By unpacking the balloon (same mass in a larger volume), the release system lowers the balloon's average density and allows it to rise.

(C) Your friend in the balloon lowers a long tube over the side of the balloon and its end dips into a huge container of lemonade 100 feet below. He begins to suck on the tube and lemonade begins rising toward his mouth. Can he succeed in drinking lemonade this way? BRIEFLY justify your answer.

Answer: No. Atmospheric pressure cannot support that tall a column of water, even if he removes all the air inside the tube.
Why: When you friend sucks on the tube, he is removing air from within that tube and dropping the pressure above the lemonade in the tube. Atmospheric pressure around the lemonade then pushes the lemonade up the tube toward his mouth. However, atmospheric pressure can only support a column of lemonade that is about 30 feet (or more accurately about 10 meters) tall. Your friend is far too high up for atmospheric pressure to be able to lift the lemonade to his mouth.

(D) Your friend then pours a stream of water out of a cup onto one of the picnic tables far below. Neglect any air resistance or friction. Identify the transformations of energy that occur in this stream as it descends and hits the table's surface.

Answer: The water's gravitational potential energy becomes kinetic energy becomes pressure potential energy.
Why: As the water descends in the open air, its gravitational potential energy transforms into kinetic energy. But when that fast-moving water hits the tabletop, it slows rapidly and its kinetic energy becomes pressure potential energy.