Physics 1050 - How Things Work - Fall 2012 Midterm Examination 2 Solutions

Midterm Examination 2 Solutions

I am so pressed for time right now, that I may not manage to post solution videos before Monday's exam -- Lou Bloomfield, Dec 15, 2012 at 11:45am

Your friend is a famous sculptor and she has just completed the first sculpture in a series called "Stand Up Straight." Alas, whenever she sets the sculpture upright on the horizontal concrete floor and lets go of it, the sculpture tips over. Why won't this sculpture remain upright?

(A) The sculpture can increase its total potential energy by tipping. [9.2% picked]

(B) The sculpture can decrease its total energy by tipping. [3.8% picked]

(D) The sculpture can increase its total energy by tipping. [0.8% picked]

Answer: (C) The sculpture can decrease its total potential energy by tipping. [85.8% picked]

You are moving a bucket of water around in a horizontal circle at a steady speed and the bucket remains 4 feet above the ground. A friend looking down on you from a balcony sees the bucket moving counter-clockwise around you: it is north of you, west of you, south of you, east of you, north of you, and so on. Just as the bucket reaches the point exactly north of you, you let go of the bucket. In which direction does the bucket travel after you release it?

(A) Toward the northwest. [4.2% picked]

(B) Toward the west. [91.7% picked]

(D) Toward the north. [3.3% picked]

You are riding on a rollercoaster loop-the-loop and, at this moment, you are exactly at the top of the loop (you have gone up one side of the loop and are about to start down the other side of the loop). Both you and the car you are seated in are upside down, yet you still feel pressed into your seat. At this moment, you are accelerating

(A) exactly downward (toward the center of the Earth) at greater than the acceleration due to gravity. [91.7% picked]

(B) forward (in the direction of your motion). [0.8% picked]

(D) exactly downward (toward the center of the Earth) at less than the acceleration due to gravity. [6.7% picked]

Answer: (A) exactly downward (toward the center of the Earth) at greater than the acceleration due to gravity. [91.7% picked]

A pitcher throws a spinning baseball toward home plate and the baseball curves toward the pitcher's right. Why did the baseball curve?

(A) The spinning baseball had angular momentum toward the left and the ball reacted to this angular momentum by pushing itself toward the right. [9.2% picked]

(B) The pitcher pushed the spinning baseball toward the right and the pitcher's rightward force caused the baseball to accelerator toward the right. [6.3% picked]

(C) The spinning baseball had angular momentum toward the right and that angular momentum bent its path toward the right. [27.5% picked]

(D) The spinning baseball deflected the passing air toward the pitcher's left. [55.4% picked]

Answer: (D) The spinning baseball deflected the passing air toward the pitcher's left. [55.4% picked]

To win a prize at the fair, you must tip over a heavy jug by hitting it with a 1-kg object. Your best bet is to transfer as much momentum as possible to the jug, so which of the following 1-kg objects should you throw at the jug?

(A) An object of medium hardness that is bounces very well. [60.4% picked]

(B) An object of medium hardness that does not bounce at all. [17.9% picked]

(D) A very hard object that bounces a medium amount. [18.8% picked]

Answer: (A) An object of medium hardness that is bounces very well. [60.4% picked]

A astronaut is orbiting the Earth inside the International Space Station and he is hovering in the middle of the cabin. The net force on that astronaut is

(A) zero and he is not accelerating. [22.1% picked]

(B) zero, but his acceleration is changing as his path bends around the Earth. [24.6% picked]

(D) zero, but his velocity is changing as his path bends around the Earth. [18.8% picked]

Answer: (C) equal to his weight and he is accelerating downward (toward the center of the Earth). [34.2% picked]

A man and a woman ski off the flat, horizontal roof of a maintenance shed side-by-side at the same time and in the same direction. They land safely on a flat, horizontal field 10 feet below the roof. He weighs twice as much as she does, but she is traveling twice as fast when the two of them leave the shed's roof. In this situation,

(A) he lands on the field much soon than she does, but she lands much farther from the shed than he does. [0.8% picked]

(B) he lands on the field much soon than she does, but they land at approximately the same distance from the shed. [0.4% picked]

(C) they land on the field at approximately the same time, but she lands much farther from the shed as he does. [94.6% picked]

(D) they land on the field at approximately the same time and the same distance from the shed. [4.2% picked]

Answer: (C) they land on the field at approximately the same time, but she lands much farther from the shed as he does. [94.6% picked]

An airplane is flying westward at a constant horizontal velocity. The air was calm before the airplane passes through it. After the airplane passes through the air, in which direction is the air moving?

(A) Downward (toward the center of the earth). [77.9% picked]

(B) Eastward and downward (at an angle between eastward and downward). [6.7% picked]

(D) The air does not move on average. [5.4% picked]

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) Just before you touch the trampoline for the first time. [0.0% picked]

(B) When you touch the trampoline for the first time. [1.7% picked]

(D) When you reach your lowest point on the trampoline in the middle of your first bounce. [2.1% picked]

Answer: (C) When you pass through equilibrium on the trampoline for the first time. [96.3% picked]

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 steadily up to the 8th floor. So they carry the nozzle end of the same fire hose to the top of a 4-story-tall ladder and again point the nozzle upward. How does the speed of the water as it emerges from the nozzle near the top of the ladder compare to its speed when it emerged from the nozzle near the ground?

(A) The water emerges from the nozzle at the same speed in both cases. [29.2% picked]

(B) The water emerging from the nozzle near the top of the ladder travels faster. [3.8% picked]

(D) The water emerging from the nozzle near the top of the ladder travels more slowly. [65.8% picked]

Answer: (D) The water emerging from the nozzle near the top of the ladder travels more slowly. [65.8% picked]

Why does a beanbag bounce poorly when dropped on a concrete sidewalk and bounce well when dropped on a trampoline?

(A) Hard surfaces do not store and return energy well, so objects do not bounce well from hard concrete sidewalks. Soft surfaces store and return energy efficiently, so objects bounce well from soft trampolines. [30.4% picked]

(B) During their collision, the beanbag transfers much less energy to the soft trampoline than to the hard sidewalk. The beanbag's remaining energy causes the beanbag to bounce upward from the trampoline. [6.3% picked]

(C) When the beanbag collides with the hard sidewalk, the beanbag is softer, and receives and wastes nearly all of the collision energy. When the beanbag collides with the soft trampoline, the trampoline is softer, and receives and returns nearly all of the collision energy. [57.5% picked]

(D) During their collision, the beanbag transfers much less momentum to the soft trampoline than to the hard sidewalk. The beanbag's remaining momentum causes the beanbag to bounce upward from the trampoline. [5.4% picked]

Answer: (C) When the beanbag collides with the hard sidewalk, the beanbag is softer, and receives and wastes nearly all of the collision energy. When the beanbag collides with the soft trampoline, the trampoline is softer, and receives and returns nearly all of the collision energy. [57.5% picked]

You're bored with college and run away to join the circus. After learning how to juggle 7 balls at once, you need a bigger challenge. You try juggling while balancing a chair on the top of your head. Only one leg of the chair touches your head, yet you manage to keep it from falling by continuously moving the chair's contact point with your head underneath (vertically below) the chair's

(A) center of gravity. [71.3% picked]

(B) geometrical center. [0.0% picked]

(D) center of mass. [27.9% picked]

If you drop a penny off the Empire State Building, it will accelerate for only about 2 seconds and then coast all the way to the sidewalk at a constant terminal velocity of about 25 miles per hour downward. The penny is evidently experiencing an upward force that is equal in amount to the penny's weight. What force is pushing the penny upward?

(A) A buoyant force. [17.1% picked]

(B) A lift force. [11.3% picked]

(D) A viscous drag force (there is no turbulent wake above the penny). [15.0% picked]

Answer: (C) A pressure drag force (there is a turbulent wake above the penny). [56.3% picked]

A carbon dioxide molecule has more mass than the average air molecule. You blow two large soap bubbles, one filled with ordinary air and one filled with carbon dioxide. The bubbles look identical and they are both at room temperature. How do the bubbles compare?

(A) They have equal weights, they contain the same number of gas particles, and their pressures are equal, but the carbon dioxide bubble experiences less buoyant force than does the air bubble. [14.6% picked]

(B) They experience the same buoyant force, they have equal weights, and they contain the same number of gas particles, but the carbon dioxide bubble has a higher pressure inside it than does the air bubble. [9.6% picked]

(C) Their pressures are equal, they experience the same buoyant force, and they have equal weights, but the carbon dioxide bubbles contains fewer gas particles than the air bubble. [10.0% picked]

(D) They contain the same number of gas particles, their pressures are equal, and they experience the same buoyant force, but the carbon dioxide bubble weighs more than the air bubble. [65.4% picked]

Answer: (D) They contain the same number of gas particles, their pressures are equal, and they experience the same buoyant force, but the carbon dioxide bubble weighs more than the air bubble. [65.4% picked]

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. [2.5% picked]

(B) increased, but the density inside the bottle has stayed the same. [6.3% picked]

(D) increased and the density inside the bottle has decreased. [88.3% picked]

Answer: (D) increased and the density inside the bottle has decreased. [88.3% picked]

You are a stunt driver for a movie. Your car is motionless at the start of your stunt and both your actual weight and your apparent weight (your feeling of weight) are normal. You then drive your car at 100 mph up a ramp, over a wide river, and down a second ramp on the far side the river. While you are midway between the two ramps, your weight is [neglect any effects due to the air]

(A) still normal and your apparent weight is normal. [3.3% picked]

(B) zero, but your apparent weight is normal. [0.4% picked]

(D) still normal, but your apparent weight is zero. [91.7% picked]

You are playing miniature golf with friends and, as part of one hole, your golf ball rolls on a series of different ramps. All of the ramps have flat surfaces, but some ramps are steeper than others. The steeper the ramp on which the ball rolls,

(A) the greater the ball's acceleration. [84.2% picked]

(B) the greater the ball's speed. [0.8% picked]

(C) the smaller the ball's acceleration when it is heading up the ramp and the greater the ball's acceleration when it is heading down the ramp. [10.8% picked]

(D) the smaller the ball's speed when it is heading up the ramp and the greater the ball's speed when it is heading down the ramp. [4.2% picked]

The dry cleaner returns your cloths protected by large, very thin plastic bags. You seal two of these bags with tape and fill one bag with air and the other bag with natural gas (methane). You inflate them until they are full but not taut (the plastic is still limp). The bags are identical in every way except for the difference in gas (air versus methane). When you let go of the two bags, the air-filled bag descends and the methane-filled bag rises. The bags have equal

(A) densities (mass per cubic meter), but the methane-filled bag experiences a large buoyant force than the air-filled bag. [6.7% picked]

(B) particle densities (number of gas particles per cubic meter), but the methane-filled bag weighs less than the air-filled bag. [66.3% picked]

(C) particle densities (number of gas particles per cubic meter), but the methane-filled bag experiences a larger buoyant force than the air-filled bag. [16.7% picked]

(D) densities (mass per cubic meter), but the methane-filled bag weighs less than the air-filled bag. [10.0% picked]

Answer: (B) particle densities (number of gas particles per cubic meter), but the methane-filled bag weighs less than the air-filled bag. [66.3% picked]

You partially fill a rigid plastic container with hot soup, seal the container with a lid, and put the container in the refrigerator. Along with the soup, you trapped hot air in the rigid container. When you open the lid of the cold container, a few hours later, air rushes

(A) into the container. As the hot air you trapped in the rigid container cooled, its pressure decreased to less than atmospheric pressure. [62.1% picked]

(B) out of the container. The hot air you trapped in the rigid container was low density air and, as it cooled, its density increased. [15.0% picked]

(C) out of the container. The hot air you trapped in the rigid container was high pressure air and it escapes from the now cooled container. [15.8% picked]

(D) into the container. The hot air you trapped in the rigid container was high density air and, as it cooled, its density decreased. [7.1% picked]

Answer: (A) into the container. As the hot air you trapped in the rigid container cooled, its pressure decreased to less than atmospheric pressure. [62.1% picked]

A "RIF" baseball is identical to an official major league baseball in every way except that the RIF ball's surface is less stiff (it dents more easily) than a major league baseball. Suppose a 10-year-old boy is hit in the head by a pitched baseball. How does the choice of ball affect the impact?

(A) The two balls transfer approximately equal momentums to the boy, but the RIF ball takes less time to transfer that momentum and exerts a larger force on the boy. [2.1% picked]

(B) The RIF ball transfers significantly more momentum to the boy. [1.7% picked]

(D) The two balls transfer approximately equal momentums to the boy, but the RIF ball takes longer to transfer that momentum and exerts a smaller force on the boy. [94.2% picked]

Answer: (D) The two balls transfer approximately equal momentums to the boy, but the RIF ball takes longer to transfer that momentum and exerts a smaller force on the boy. [94.2% picked]

You are working in a pizza parlor. You toss a spinning disk of pizza dough into the air. As the dough stretches outward and the flying disk becomes wider, what happens to the disk's angular velocity?

(A) It remains constant because no torque acts on the disk. [0.8% picked]

(B) It remains constant because angular velocity is conserved. [0.4% picked]

(D) It decreases. [93.3% picked]

You are riding a carousel. You and the toy horse you are sitting on are traveling in a circle as the carousel turns steadily. The force that the horse exerts on you is directed

(A) exactly upward (straight up toward the sky). [59.6% picked]

(B) at an angle between upward and away from the center of the carousel. [4.2% picked]

(D) at an angle between upward and forward (in the direction of your motion). [5.8% picked]

Answer: (C) at an angle between upward and toward the center of the carousel. [30.0% picked]

You are competing in the 400-meter sprint on an oval track. As you round the final turn at world record pace, your path is curving toward your left and you find yourself leaning toward the left. Why can't you complete the turn safely if you don't lean and instead remain upright during the turn?

(A) Your center of gravity will not be above your base of support on the track and you will not have static stability. [29.2% picked]

(B) Your center of gravity will be too high above the ground and you will have too much energy to complete the turn safely. [11.3% picked]

(C) The track's force on your feet will produce a torque on you about your center of mass and tip you over. [53.8% picked]

(D) The track's force on your feet will make you accelerate toward your center of mass and tip over. [5.4% picked]

Answer: (C) The track's force on your feet will produce a torque on you about your center of mass and tip you over. [53.8% picked]

You are trying to set the world's record for drinking water through the tallest drinking straw. What could you do to have the best chance of setting this record?

(A) Use the narrowest drinking straw you can find. [1.7% picked]

(B) Use the widest drinking straw you can fit in your mouth. [0.8% picked]

(D) Go to the lowest altitude available, preferably sea level or below. [94.2% picked]

Answer: (D) Go to the lowest altitude available, preferably sea level or below. [94.2% picked]

A gymnasium has a large ventilation duct near its ceiling. When air that was moving at uniform velocity and pressure through a straight section of duct enters a bend in the duct and it curves, its pressure is greatest at the [neglect any effects due to viscosity or turbulence]

(A) inside of the bend and its speed is greatest at the outside of the bend. [6.7% picked]

(B) outside of the bend and its speed is greatest at the inside of the bend. [86.3% picked]

(D) inside of the bend and its speed is greatest at the inside of the bend. [0.0% picked]

Answer: (B) outside of the bend and its speed is greatest at the inside of the bend. [86.3% picked]

As a rocket takes off from its launch pad, a plume of flaming exhaust gas squirts out of its engine and scorches the launch pad. What is exerting the upward force on the rocket that causes the rocket to accelerate upward?

(A) The launch pad is pushing the rocket upward. [2.9% picked]

(B) The exhaust gas is pushing the rocket upward. [93.3% picked]

(D) The air is pushing the rocket upward. [1.7% picked]

You drop an unopened metal can of soup and it lands bottom-first on the floor. When you inspect the can, you see that has dented outward in places. Where did it dent outward and why?

(A) The soup pressure near the bottom of the can increased dramatically as the soup transferred its momentum suddenly to the floor and that pressure dented the can outward near its bottom. [35.0% picked]

(B) The floor pushed the bottom of the can upward extremely hard on impact and the momentum of that upward force dented the can outward near its top. [22.9% picked]

(C) The impact between the floor and the bottom of the can buckled the can outward near its middle (halfway between its top and bottom). [2.9% picked]

(D) The soup bounced upward after the can hit the floor and when the soup hit the top of the can, it dented the top of the can outward. [38.8% picked]

Answer: (A) The soup pressure near the bottom of the can increased dramatically as the soup transferred its momentum suddenly to the floor and that pressure dented the can outward near its bottom. [35.0% picked]

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) momentum increases. [0.4% picked]

(B) particle density decreases. [0.8% picked]

(D) density decreases. [1.7% picked]

A simple squirt gun is a hand-operated toy that shoots a stream of water. As your finger pushes on a moving part of the gun, water emerges from the gun's nozzle and shoots straight upward. As you push harder on the gun's moving part, the stream of water goes higher because you are

(A) increasing the amount of water that is moving in the stream and therefore pushing it higher up into the air. [2.9% picked]

(B) doing more work on each drop of water flowing through the gun and each drop thus has more total energy to turn into gravitational potential energy as it rises. [47.9% picked]

(C) increasing the water's gravitational kinetic energy while it is inside the gun and it can turn that gravitational kinetic energy into more gravitational potential energy as it rises. [4.6% picked]

(D) increasing the pressure that the stream of water has as it travels upward after leaving the gun's nozzle. [44.6% picked]

Answer: (B) doing more work on each drop of water flowing through the gun and each drop thus has more total energy to turn into gravitational potential energy as it rises. [47.9% picked]

A building with two floors has simple plumbing with no moving parts. If you try the showers on each floor, you will find that [neglect any effects due to viscosity or turbulence]

(A) the total energy per drop and the pressure before the shower head are the same on both floors, however, the upper floor has slower moving water after the shower head. [2.5% picked]

(B) the upper floor has less total energy per drop, less pressure before the shower head, and slower moving water after the shower head. [19.2% picked]

(C) the total energy per drop is the same on both floors and that both floors have the same pressure before the shower head and produce equally fast moving water after the shower head. [1.3% picked]

(D) the total energy per drop is the same on both floors, however, the upper floor has less pressure before the shower head and slower moving water after the shower head. [75.8% picked]

Answer: (D) the total energy per drop is the same on both floors, however, the upper floor has less pressure before the shower head and slower moving water after the shower head. [75.8% picked]