Midterm Examination 2 Solutions

Problem 1:

You are on a 3rd floor hotel balcony, 10 meters above the bar. You lower a long straw into a pitcher of diet soda (essentially pure water) and find that you can just barely suck the soda up to your mouth. You move the straw over to pitcher filled with full-calorie soda (essentially sugar water, with a density that is 10% more than pure water). Now what happens when you begin sucking?

(A) You can suck the full-calorie soda up to your mouth more easily than you did diet soda, because the full-calorie soda has more inertia. [0.4% picked]

(B) You are unable to suck the full-calorie soda up to your mouth. [79.2% picked]

(C) You can suck the full-calorie soda up to your mouth with the same difficulty as when you were sucking on the diet soda. [16.5% picked]

(D) You can suck the full-calorie soda up to your mouth more easily than you did diet soda, because the full-calorie soda has more momentum. [2.5% picked]

Answer: (B) You are unable to suck the full-calorie soda up to your mouth. [79.2% picked]

Why: Video Discussion

Problem 2:

A satellite is traveling around the earth in a circular orbit. It briefly fires its rocket engine to increase its speed in the forward direction; it is suddenly going faster but its direction of travel didn't change. As a result of this speed increase, the orbiting satellite's average distance from the center of the earth

(A) increases. [29.7% picked]

(B) decreases. [7.6% picked]

(C) remains unchanged, but the orbit is no longer circular. [31.8% picked]

(D) remains unchanged and the orbit continues to be circular. [25.8% picked]

Answer: (A) increases. [29.7% picked]

Why: Video Discussion

Problem 3:

On a cold morning, you seal your empty plastic water bottle and thus trap the air inside it. Later in the day, the bottle and its contents are much warmer and the sides of the bottle are now bowed outward. As a result of these changes, the air pressure inside the bottle has

(A) stayed the same, but the density inside the bottle has decreased. [7.2% picked]

(B) increased, but the density inside the bottle has stayed the same. [32.6% picked]

(C) increased and the density inside the bottle has decreased. [42.8% picked]

(D) increased and the density inside the bottle has increased. [14.0% picked]

Answer: (C) increased and the density inside the bottle has decreased. [42.8% picked]

Why: Video Discussion

Problem 4:

As water in steady state flows straight down in a vertical pipe with a uniform diameter, that water transforms its

(A) kinetic energy into gravitational potential energy. [0.4% picked]

(B) gravitational potential energy into pressure potential energy. [79.2% picked]

(C) gravitational potential energy into kinetic energy. [7.2% picked]

(D) pressure potential energy into gravitational potential energy. [13.1% picked]

Answer: (B) gravitational potential energy into pressure potential energy. [79.2% picked]

Why: Video Discussion

Problem 5:

A hydrogen-filled dirigible (a huge blimp or elongated balloon) floats in air because the hydrogen-filled dirigible contains

(A) the same number of particles as the air it displaces, but each hydrogen molecule weighs less than the average air molecule. [93.2% picked]

(B) faster-moving hydrogen molecules, which use their kinetic energies to displace the force of gravity. [0.4% picked]

(C) more particles than the air it displaces and these particles push upward more strongly on the top surface of the dirigible. [2.1% picked]

(D) fewer particles than the air it displaces. [3.8% picked]

Answer: (A) the same number of particles as the air it displaces, but each hydrogen molecule weighs less than the average air molecule. [93.2% picked]

Why: Video Discussion

Problem 6:

As part of your new circus routine, you jump off a high platform onto a trampoline. You bounce up and down spectacularly before eventually settling at equilibrium on the trampoline. When during your descent from the high platform do you first stop accelerating downward?

(A) When you pass through equilibrium on the trampoline for the first time. [66.9% picked]

(B) When you reach your lowest point on the trampoline in the middle of your first bounce. [22.0% picked]

(C) Just before you touch the trampoline for the first time. [1.3% picked]

(D) When you touch the trampoline for the first time. [9.3% picked]

Answer: (A) When you pass through equilibrium on the trampoline for the first time. [66.9% picked]

Why: Video Discussion

Problem 7:

Water is flowing at constant velocity through a straight horizontal pipe. It is experiencing two forces: a forward force due to a difference in pressure at the two ends of the pipe and a backward force due to the water's viscosity and friction with the stationary pipe. How do the amounts of those two forces compare?

(A) The forward force is slightly smaller than the backward force. [0.4% picked]

(B) The forward force is slightly larger than the backward force. [8.9% picked]

(C) The forward force is twice as large as the backward force. [6.4% picked]

(D) The two forces are equal in amount. [80.9% picked]

Answer: (D) The two forces are equal in amount. [80.9% picked]

Why: Video Discussion

Problem 8:

You are drying your hair and you have put a diffuser (an anti-nozzle) on the end of the dryer. As it goes through the diffuser, the air slows down and its

(A) particle density decreases. [18.2% picked]

(B) density decreases. [18.6% picked]

(C) momentum increases. [0.0% picked]

(D) pressure increases. [58.1% picked]

Answer: (D) pressure increases. [58.1% picked]

Why: Video Discussion

Problem 9:

You're standing on the middle of the Ruffner Hall/Bavaro 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 speed is greatest when you are

(A) just about to reach the top or bottom of the bounce. [4.2% picked]

(B) just about to pass through equilibrium on your way upward or downward. [21.2% picked]

(C) passing through equilibrium on your way upward or downward. [69.1% picked]

(D) at the top or bottom of the bounce. [4.2% picked]

Answer: (C) passing through equilibrium on your way upward or downward. [69.1% picked]

Why: Video Discussion

Problem 10:

A block of wood is floating easily on the surface of a lake. The sun comes out and warms the air above the lake. The warming air's density decreases and the block of wood

(A) continues to float just as it did before. [1.7% picked]

(B) moves upward slightly and floats a little less deep in the water. [26.7% picked]

(C) moves downward slightly and floats a little deeper in the water. [71.6% picked]

(D) sinks to the bottom of the lake. [0.0% picked]

Answer: (C) moves downward slightly and floats a little deeper in the water. [71.6% picked]

Why: Video Discussion

Problem 11:

When you drop a beanbag on cement, the bag doesn't bounce. Suppose you are playing tennis with a beanbag instead of a tennis ball. How should you adjust your tennis racket to make the beanbag travel as fast as possible after you hit it with the racket?

(A) You should add mass to the racket so that it carries more momentum. [6.4% picked]

(B) You should loosen the strings so that the racket surface is like a soft trampoline. [62.7% picked]

(C) You should replace the strings with a rigid carbon-fiber plate so that the racket surface acts like cement. [0.8% picked]

(D) You should tighten the strings so that the racket surface is like a firm trampoline. [28.4% picked]

Answer: (B) You should loosen the strings so that the racket surface is like a soft trampoline. [62.7% picked]

Why: Video Discussion

Problem 12:

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

(A) less than that of oil but more than that of vinegar. [11.9% picked]

(B) more than that of either vinegar or oil. [0.0% picked]

(C) less than that of vinegar but more than that of oil. [87.3% picked]

(D) less than that of either vinegar or oil. [0.4% picked]

Answer: (C) less than that of vinegar but more than that of oil. [87.3% picked]

Why: Video Discussion

Problem 13:

The body of your car is supported by springs. When you put a 100-pound box of books into the trunk, the body's average height above the ground decreases by 1 inch. If you add a second 100-pound box of books to the trunk, the body's average height above the ground will decrease by an additional

(A) 3 inches. [0.4% picked]

(B) 4 inches. [1.7% picked]

(C) 1 inch. [86.0% picked]

(D) 2 inches. [9.3% picked]

Answer: (C) 1 inch. [86.0% picked]

Why: Video Discussion

Problem 14:

When you launch a toy water rocket, compressed air inside the rocket pushes water out of the rocket's tail as "exhaust" and the rocket blasts forward at high speed. Normally, you fill the rocket's hollow shell half-full with water and half-full with compressed air. Eliminating the water would allow you to put twice as much compressed air in the rocket and would double the rocket's stored energy. However, the air-filled rocket will barely move forward when you launch it because its air exhaust

(A) can't push against the launch pad the way water exhaust can, so that the launch pad won't be able to push the rocket forward as hard. [12.7% picked]

(B) will not travel as fast as the water. [0.4% picked]

(C) has very little weight and therefore can't push very hard on the rocket. [16.9% picked]

(D) will have almost no mass and therefore carry almost no backward momentum. [67.8% picked]

Answer: (D) will have almost no mass and therefore carry almost no backward momentum. [67.8% picked]

Why: Video Discussion

Problem 15:

You are watering the garden with a hose and nozzle. You are holding the nozzle horizontally in order to spray a distant flower. As the water flows through that nozzle, it converts

(A) gravitational potential energy into kinetic energy. [2.1% picked]

(B) pressure potential energy into kinetic energy. [92.8% picked]

(C) kinetic energy into gravitational potential energy. [2.1% picked]

(D) kinetic energy into pressure potential energy. [3.0% picked]

Answer: (B) pressure potential energy into kinetic energy. [92.8% picked]

Why: Video Discussion

Problem 16:

An apple sits motionless on a table. The amount of momentum that apple is transferring to the table each second is

(A) the apple's mass times 1 second. [16.5% picked]

(B) the apple's weight divided by the apple's mass. [1.7% picked]

(C) zero. [29.7% picked]

(D) the apple's weight times 1 second. [50.4% picked]

Answer: (D) the apple's weight times 1 second. [50.4% picked]

Why: Video Discussion

Problem 17:

The maximum speed of a rocket-propelled spaceship with an exhaust speed of 5000 mph is

(A) 10000 mph. [1.7% picked]

(B) limited only by the ratio of fuel to spaceship. [65.3% picked]

(C) 5000 mph. [25.0% picked]

(D) 2500 mph. [0.4% picked]

Answer: (B) limited only by the ratio of fuel to spaceship. [65.3% picked]

Why: Video Discussion

Problem 18:

To win a prize at the fair, you must toss a basketball into a stiff fruit basket and have the ball remain in that basket. The basket is tipped toward you and nailed to the wall. Its lower lip has just enough uphill slope to keep a stationary ball from rolling out. Each time you throw the ball into the basket, the ball bounces and rolls onto the floor. The conserved quantity that the basketball is unable to get rid of in order to stay in the basket is

(A) momentum. [14.8% picked]

(B) kinetic energy. [4.2% picked]

(C) potential energy. [3.0% picked]

(D) energy. [77.5% picked]

Answer: (D) energy. [77.5% picked]

Why: Video Discussion

Problem 19:

When you ride a bicycle, the ground exerts an upward support force on the wheel. When you turn the bicycle, the ground also exerts a horizontal frictional force on the wheel. Leaning the bicycle toward the inside of a turn keeps you from tipping over because the overall force that the ground exerts on the wheel (the sum of the support force and the frictional force)

(A) points directly upward so that you don't fall downward. [3.0% picked]

(B) is exactly zero and causes no acceleration of your center of mass. [10.2% picked]

(C) points exactly at right angles to your center of mass and exerts a torque on you that keeps you from falling over. [9.3% picked]

(D) points directly at your combined center of mass and produces zero torque on you and the bicycle. [77.1% picked]

Answer: (D) points directly at your combined center of mass and produces zero torque on you and the bicycle. [77.1% picked]

Why: Video Discussion

Problem 20:

You are at a barbeque and the bottle of hot sauce is almost empty. You put the cap on the bottle and swing it rapidly in a circle with its cap end pointing outward. The remaining sauce collects just inside the cap and you are able to extract enough to spice up your vegiburger. This technique works because

(A) the bottle's rapid inward acceleration leaves the hot sauce behind so that it drifts toward the cap end of the bottle. [65.7% picked]

(B) the force of the hot sauce's momentum pushes it toward the cap end of the bottle. [3.8% picked]

(C) centrifugal force pushes the hot sauce toward the cap end of the bottle. [23.3% picked]

(D) viscous forces push the hot sauce toward the cap end of the bottle. [3.0% picked]

Answer: (A) the bottle's rapid inward acceleration leaves the hot sauce behind so that it drifts toward the cap end of the bottle. [65.7% picked]

Why: Video Discussion

Problem 21:

If you're trying to increase the pressure in the water distribution system by modifying the local water tower, you should make the water tower

(A) narrower. [0.8% picked]

(B) taller. [97.5% picked]

(C) wider. [0.8% picked]

(D) shorter. [0.4% picked]

Answer: (B) taller. [97.5% picked]

Why: Video Discussion

Problem 22:

You're at the lake and watch two children jump off a dock at the same time. They both kick equally hard during their jumps, but one child jumps mostly upward while the other child jumps mostly forward. After they leave the dock,

(A) the two children reach the water at the same moment and at the same distance from the dock. [1.3% picked]

(B) the two children reach the water at the same moment and but the child who jumps forward travels farther from the dock than does the other child. [35.6% picked]

(C) the child who jumps upward reaches the water before the child who jumps forward. [1.7% picked]

(D) the child who jumps forward reaches the water before the child who jumps upward. [61.4% picked]

Answer: (D) the child who jumps forward reaches the water before the child who jumps upward. [61.4% picked]

Why: Video Discussion

Problem 23:

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

(A) 9th floor. [4.7% picked]

(B) 6th floor. [3.0% picked]

(C) 8th floor. [46.6% picked]

(D) 10th floor. [40.7% picked]

Answer: (C) 8th floor. [46.6% picked]

Why: Video Discussion

Problem 24:

You are floating along in a hot air balloon. You look up and notice that the bottom of the balloon is open. Apart from a few molecules that diffuse out, hot air remains inside the balloon despite this opening because

(A) hot air has more inertia than cold air and doesn't accelerate easily. [0.0% picked]

(B) the air pressure just inside the opening is the same as the air pressure just outside that opening. [95.8% picked]

(C) the propane burner located below the opening keeps pushing the hot air back into the balloon [0.4% picked]

(D) hot air has a lower pressure than cold air, so hot air is drawn into the balloon by the partial vacuum inside it. [3.8% picked]

Answer: (B) the air pressure just inside the opening is the same as the air pressure just outside that opening. [95.8% picked]

Why: Video Discussion

Problem 25:

You are riding on a playground swing. As you swing forward, there is a moment when you are directly below the pivot that supports the swing. At that moment, your speed is momentarily constant and you are

(A) accelerating backward. [1.3% picked]

(B) accelerating upward. [86.9% picked]

(C) accelerating forward. [5.5% picked]

(D) not accelerating at all. [6.4% picked]

Answer: (B) accelerating upward. [86.9% picked]

Why: Video Discussion

Problem 26:

You are bicycling at constant velocity along a horizontal road and you are wearing a backpack. As you bicycle, you are doing

(A) (positive) work on the backpack and it is doing (positive) work on you. [4.7% picked]

(B) (positive) work on the backpack and it is doing negative work on you. [26.3% picked]

(C) zero work on the backpack and it is doing zero work on you. [66.5% picked]

(D) negative work on the backpack and it is doing (positive) work on you. [0.8% picked]

Answer: (C) zero work on the backpack and it is doing zero work on you. [66.5% picked]

Why: Video Discussion

Problem 27:

You drive too fast over a small hill and your car's wheels lose contact with the road. While your car is not touching the road, you are

(A) accelerating downward and you feel lighter than you do while at rest. [90.7% picked]

(B) accelerating downward and you feel heavier than you do at rest. [4.7% picked]

(C) not accelerating, but you feel lighter than you do while at rest. [3.8% picked]

(D) not accelerating, but you feel heavier than you do at rest. [0.8% picked]

Answer: (A) accelerating downward and you feel lighter than you do while at rest. [90.7% picked]

Why: Video Discussion

Problem 28:

You're filming a movie and you arrange for two unoccupied cars to drive horizontally off a tall cliff. The Mercedes sedan weighs twice as much as the Mini Cooper, but the Mini Cooper is traveling at twice the velocity of the Mercedes when the two cars careen off the cliff side-by-side. In this situation, the two cars hit the level ground below the cliff

(A) at approximately the same distance from the cliff, but the Mercedes sedan hits much sooner than the Mini Cooper. [0.4% picked]

(B) at approximately the same time, but the Mercedes sedan hits considerably farther from the cliff than the Mini Cooper. [1.7% picked]

(C) at approximately the same time and at the same distance from the cliff. [14.0% picked]

(D) at approximately the same time, but the Mini Cooper hits considerably farther from the cliff than the Mercedes sedan. [83.9% picked]

Answer: (D) at approximately the same time, but the Mini Cooper hits considerably farther from the cliff than the Mercedes sedan. [83.9% picked]

Why: Video Discussion

Problem 29:

A bicycle is remarkably stable while it's moving forward because, when you start to tip over,

(A) the bicycle's base of support becomes much wider so that a torque from the ground returns it to an upright orientation. [0.8% picked]

(B) the bicycle spontaneously steers so as to reduce your kinetic energy. [0.0% picked]

(C) the bicycle spontaneously steers so that its wheels drive under your center of gravity. [96.2% picked]

(D) the bicycle spontaneously steers so as to reduce your gravitational potential energy. [3.0% picked]

Answer: (C) the bicycle spontaneously steers so that its wheels drive under your center of gravity. [96.2% picked]

Why: Video Discussion

Problem 30:

You roll a baseball off the top of a tall building and it hits the ground 4 seconds later. When the baseball has fallen for only 2 seconds, how far is it above the ground? [Note: neglect any effects due to the air.]

(A) It is slightly more than 1/2 of the building's height above the ground. [11.4% picked]

(B) It is 1/2 of the building's height above the ground. [8.1% picked]

(C) It is 3/4 of the building's height above the ground. [69.9% picked]

(D) It is slightly less than 1/2 of the building's height above the ground. [8.5% picked]

Answer: (C) It is 3/4 of the building's height above the ground. [69.9% picked]

Why: Video Discussion