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
Problem 1:
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?
Answer: (C) The sculpture can decrease its total potential energy by tipping. [85.8% picked]
Why: TBA
Problem 2:
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?
Answer: (B) Toward the west. [91.7% picked]
Why: TBA
Problem 3:
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
Answer: (A) exactly downward (toward the center of the Earth) at greater than the acceleration due to gravity. [91.7% picked]
Why: TBA
Problem 4:
A pitcher throws a spinning baseball toward home plate and the baseball curves toward the pitcher's right. Why did the baseball curve?
Answer: (D) The spinning baseball deflected the passing air toward the pitcher's left. [55.4% picked]
Why: TBA
Problem 5:
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?
Answer: (A) An object of medium hardness that is bounces very well. [60.4% picked]
Why: TBA
Problem 6:
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
Answer: (C) equal to his weight and he is accelerating downward (toward the center of the Earth). [34.2% picked]
Why: TBA
Problem 7:
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,
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]
Why: TBA
Problem 8:
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?
Answer: (A) Downward (toward the center of the earth). [77.9% picked]
Why: TBA
Problem 9:
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?
Answer: (C) When you pass through equilibrium on the trampoline for the first time. [96.3% picked]
Why: TBA
Problem 10:
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?
Answer: (D) The water emerging from the nozzle near the top of the ladder travels more slowly. [65.8% picked]
Why: TBA
Problem 11:
Why does a beanbag bounce poorly when dropped on a concrete sidewalk and bounce well when dropped on a trampoline?
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]
Why: TBA
Problem 12:
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
Answer: (A) center of gravity. [71.3% picked]
Why: TBA
Problem 13:
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?
Answer: (C) A pressure drag force (there is a turbulent wake above the penny). [56.3% picked]
Why: TBA
Problem 14:
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?
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]
Why: TBA
Problem 15:
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
Answer: (D) increased and the density inside the bottle has decreased. [88.3% picked]
Why: TBA
Problem 16:
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]
Answer: (D) still normal, but your apparent weight is zero. [91.7% picked]
Why: TBA
Problem 17:
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,
Answer: (A) the greater the ball's acceleration. [84.2% picked]
Why: TBA
Problem 18:
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
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]
Why: TBA
Problem 19:
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
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]
Why: TBA
Problem 20:
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?
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]
Why: TBA
Problem 21:
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?
Answer: (D) It decreases. [93.3% picked]
Why: TBA
Problem 22:
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
Answer: (C) at an angle between upward and toward the center of the carousel. [30.0% picked]
Why: TBA
Problem 23:
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?
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]
Why: TBA
Problem 24:
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?
Answer: (D) Go to the lowest altitude available, preferably sea level or below. [94.2% picked]
Why: TBA
Problem 25:
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]
Answer: (B) outside of the bend and its speed is greatest at the inside of the bend. [86.3% picked]
Why: TBA
Problem 26:
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?
Answer: (B) The exhaust gas is pushing the rocket upward. [93.3% picked]
Why: TBA
Problem 27:
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?
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]
Why: TBA
Problem 28:
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
Answer: (C) pressure increases. [97.1% picked]
Why: TBA
Problem 29:
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
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]
Why: TBA
Problem 30:
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]
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]
Why: TBA