Midterm Examination 1 Solutions

Problem 1:

You are tossing popcorn straight up and catching it in your mouth. At the moment each piece of popcorn reaches its peak height, its velocity is

(A) zero and its acceleration is zero. [3.4% picked]

(B) zero and its acceleration is downward. [94.7% picked]

(C) downward and its acceleration is zero. [1.5% picked]

(D) downward and its acceleration is downward. [0.4% picked]

Answer: (B) zero and its acceleration is downward. [94.7% picked]

Why: Video Discussion

Problem 2:

You are riding on a playground merry-go-round and accidently let go of your water bottle. Neglecting gravity, in which direction does your bottle travel?

(A) Your bottle continues straight ahead, in the direction it was heading when you let go of it. [85.0% picked]

(B) Your bottle travels outward, directly away from center of the merry-go-round. [4.5% picked]

(C) Your bottle travels forward and outward, in a direction that lies between straight ahead and directly away from the center of the merry-go-round. [5.3% picked]

(D) Your bottle travels inward, directly toward the center of the merry-go-round. [5.3% picked]

Answer: (A) Your bottle continues straight ahead, in the direction it was heading when you let go of it. [85.0% picked]

Why: Video Discussion

Problem 3:

It is a beautiful summer day and the residents of a high-rise apartment building are eating dinner on their balconies. A resident accidently knocks an empty glass off a balcony that is about 80 meters (260 feet) above the cement patio. The glass would have smashed on that patio after falling for 4 seconds, however, a quick-witted resident catches the glass after it has fallen for only 2 seconds. How far above the patio was the glass when it was caught?

(A) Approximately 60 meters above the patio. [78.9% picked]

(B) Approximately 30 meters above the patio. [4.5% picked]

(C) Approximately 50 meters above the patio. [8.6% picked]

(D) Approximately 40 meters above the patio. [7.9% picked]

Answer: (A) Approximately 60 meters above the patio. [78.9% picked]

Why: Video Discussion

Problem 4:

A skateboarder rides swiftly up the edge of a bowl-shaped surface and leaps into the air. While in the air, the skateboarder flips upside and tosses the skateboard from hand to hand. The skateboarder then rides safely back down the bowl. During the time that the skateboarder and skateboard are not touching anything, one aspect of their motion that is constant is their total (or combined) [note: neglect any effects due to the air]

(A) momentum. [5.3% picked]

(B) angular velocity. [6.8% picked]

(C) velocity. [0.4% picked]

(D) angular momentum. [87.6% picked]

Answer: (D) angular momentum. [87.6% picked]

Why: Video Discussion

Problem 5:

Two puppies are fighting over a toy. Each puppy grips one end of that toy in its mouth and pulls. Suddenly, the puppy on the right pulls especially hard on the toy and moves its end of the toy to the right. The other puppy manages to keep its end of the toy from moving. Alas, the toy breaks and the game ends. Breaking the toy required energy and that energy was provided

(A) by both puppies, but most was provided by the puppy on the right. [3.0% picked]

(B) only by the puppy on the right. [89.1% picked]

(C) in equal amounts by the two puppies. [7.5% picked]

(D) only by the puppy on the left. [0.4% picked]

Answer: (B) only by the puppy on the right. [89.1% picked]

Why: Video Discussion

Problem 6:

You're at the lake and watch two children jump off a dock. They jump at the same time and at the same speed, but the boy jumps mostly upward while the girl jumps mostly forward. After they leave the dock,

(A) the two children reach the water at the same moment and but the girl travels farther from the dock than does the boy. [11.3% picked]

(B) girl reaches the water before the boy. [86.5% picked]

(C) the two children reach the water at the same moment and at the same distance from the dock. [0.8% picked]

(D) boy reaches the water before the girl. [1.5% picked]

Answer: (B) girl reaches the water before the boy. [86.5% picked]

Why: Video Discussion

Problem 7:

You are rearranging your dorm room and are now sliding your desk across the floor at constant velocity. Which of the following statements about the forces acting on the desk is correct?

(A) The amount of force that you're exerting on the desk must be more than the amount of its weight. [0.8% picked]

(B) The amount of force that you're exerting on the desk must be equal to the amount of its weight. [4.9% picked]

(C) The force that you're exerting on the desk must be equal in amount but opposite in direction to the force that the floor is exerting it. [83.1% picked]

(D) The amount of force that you're exerting on the desk must be more than the amount of force that friction is exerting on it. [11.3% picked]

Answer: (C) The force that you're exerting on the desk must be equal in amount but opposite in direction to the force that the floor is exerting it. [83.1% picked]

Why: Video Discussion

Problem 8:

When a bird perches by itself on a particular tree branch, the branch bends downward 1.0 centimeter. When a bird weighing twice as much perches by itself on that same branch, the branch bends downward approximately

(A) 1.4 centimeters. [0.4% picked]

(B) 1.0 centimeter. [0.0% picked]

(C) 2.0 centimeters. [97.7% picked]

(D) 4.0 centimeters. [1.5% picked]

Answer: (C) 2.0 centimeters. [97.7% picked]

Why: Video Discussion

Problem 9:

A pole-vaulter travels over the bar and descends deep into the soft foam cushion far below. During the decent from the bar and deep into the cushion, when does the pole-vaulter reach peak downward speed?

(A) At the moment the pole-vaulter is denting the cushion as deeply as possible for the first time. [3.8% picked]

(B) At the moment the pole-vaulter reaches equilibrium on the cushion for the first time. [74.4% picked]

(C) At the moment just before the pole-vaulter first touches the cushion. [15.8% picked]

(D) At the moment the pole-vaulter first touches the cushion. [6.0% picked]

Answer: (B) At the moment the pole-vaulter reaches equilibrium on the cushion for the first time. [74.4% picked]

Why: Video Discussion

Problem 10:

A dog and a cat jump horizontally off a wall at the same moment and soon land on the level horizontal field that extends outward from the base of the wall. The dog weighs twice as much as the cat, but the cat was moving forward horizontally twice as fast as the dog when the two animals left the wall. In this situation,

(A) both animals land at approximately the same time and at approximately the same distance from the wall. [12.0% picked]

(B) the dog lands first, but the cat lands considerably farther from the wall than the dog does. [3.8% picked]

(C) both animals land at approximately the same time, but the cat lands considerably farther from the wall than the dog does. [83.1% picked]

(D) the dog lands first, but both animals land at approximately the same distance from the wall. [1.1% picked]

Answer: (C) both animals land at approximately the same time, but the cat lands considerably farther from the wall than the dog does. [83.1% picked]

Why: Video Discussion

Problem 11:

To win a stuffed animal at the local fair, you have to toss a rigid plastic ring into a hard glass bowl and have it stay in the bowl. Unfortunately, when the ring hits the bowl, it bounces wildly and almost always falls to the ground. The ring usually bounces out of the bowl because it

(A) retains approximately all of its energy but transfers a large amount of momentum to the bowl. [88.0% picked]

(B) retains approximately all of its energy and momentum. [6.4% picked]

(C) transfers a large amount of momentum and energy to the bowl. [1.1% picked]

(D) retains approximately all of its momentum but transfers a large amount of energy to the bowl. [4.5% picked]

Answer: (A) retains approximately all of its energy but transfers a large amount of momentum to the bowl. [88.0% picked]

Why: Video Discussion

Problem 12:

You are standing in the middle of a subway car that is moving forward at constant velocity when another passenger accidently spills an enormous container of olive oil. Suddenly, the floor cannot exert any frictional forces on your feet. Because nothing else is touching you, you

(A) shift toward the back of the subway car (opposite its velocity). [52.3% picked]

(B) remain in the middle of the subway car. [40.6% picked]

(C) shift toward the front of the subway car (in the direction of its velocity). [6.8% picked]

(D) shift toward the side of the subway car (perpendicular to the direction of its velocity). [0.4% picked]

Answer: (B) remain in the middle of the subway car. [40.6% picked]

Why: Video Discussion

Problem 13:

You take off your shoes to sneak quietly into your room late at night. Unfortunately, it's too dark to see the concrete block your friend left on the floor and your big toe collides with it. The block doesn't move and your foot comes to a complete stop due to the impact. Luckily, you are wearing soft woolen socks because when your foot stops during the impact, your toe transfers

(A) less velocity to the block than it would have if you had not been wearing socks. [0.0% picked]

(B) less momentum to the block than it would have if you had not been wearing socks. [3.8% picked]

(C) the same momentum, whether or not you are wearing socks, but that transfer takes more time when you are wearing socks. [94.7% picked]

(D) less energy to the block than it would have if you had not been wearing socks. [1.5% picked]

Answer: (C) the same momentum, whether or not you are wearing socks, but that transfer takes more time when you are wearing socks. [94.7% picked]

Why: Video Discussion

Problem 14:

You are at the gym, exercising on a step machine. You have one foot on each of the machine's pedals and you move those pedals up and down as you step. The pedals always push upward on your feet, but they push upward harder on your feet while moving downward than they do while moving upward. When during this exercise is your foot transferring energy to the pedal that it is touching?

(A) As that pedal moves either upward or down. [3.4% picked]

(B) When that pedal is accelerating. [1.5% picked]

(C) As that pedal moves downward. [93.2% picked]

(D) As that pedal moves upward. [1.9% picked]

Answer: (C) As that pedal moves downward. [93.2% picked]

Why: Video Discussion

Problem 15:

A juggler tosses a club straight up. Disregarding any effects due to the air, what force or forces are acting on the club while it is above the juggler's hands?

(A) Its weight along with an upward force that steadily decreases until the club reaches its highest point. After that point, there is only the constant downward force of gravity. [6.0% picked]

(B) A steadily decreasing upward force from the moment it leaves the juggler's hands until it reaches its highest point and then a steadily increasing downward force as the club returns toward the jugglers hands. [4.5% picked]

(C) Its weight along with a steadily decreasing upward force. [1.1% picked]

(D) Its weight. [88.3% picked]

Answer: (D) Its weight. [88.3% picked]

Why: Video Discussion

Problem 16:

In the game of shuffleboard, you push plastic disks forward and then release them so that they slide across a level playing surface. Once you release a disk, you are no longer pushing and it gradually skids to a stop. Its final position determines your score. As each disk skids to a stop, what becomes of the kinetic energy it had when you released it? That energy is

(A) now thermal energy in the disk and playing surface. [97.0% picked]

(B) now gravitational potential energy in the disk. [0.4% picked]

(C) now elastic potential energy in the disk. [1.5% picked]

(D) still present in the disk, as it must be because kinetic energy is conserved and can't be created or destroyed. [1.1% picked]

Answer: (A) now thermal energy in the disk and playing surface. [97.0% picked]

Why: Video Discussion

Problem 17:

You make a sharp left turn in your car and your bobble-head toy slips off the dashboard and out the right passenger window. The toy exited the car because

(A) the car did not exert enough leftward force on the toy to make the toy accelerate with the car. [88.0% picked]

(B) the toy's weight pulled the toy out the window. [3.8% picked]

(C) the car exerted a rightward centrifugal force on the toy that pushed the toy out the window. [8.3% picked]

(D) the dashboard of the turning car tilted sharply to become a ramp and the downhill ramp force pushed the toy out the window. [0.0% picked]

Answer: (A) the car did not exert enough leftward force on the toy to make the toy one accelerate with the car. [88.0% picked]

Why: Video Discussion

Problem 18:

A cross-country skier is struggling to get up a hill, so you offer to help. As the skier passes you, you reach out with your hand and exert an uphill force of 80 N (18 pounds) on the skier. When you do this, the skier exerts

(A) a downhill force of 80 N on you. [94.7% picked]

(B) no downhill force on you at all. [1.9% picked]

(C) a downhill force of somewhat less than 80 N on you. [3.0% picked]

(D) a downhill force of somewhat more than 80 N on you. [0.4% picked]

Answer: (A) a downhill force of 80 N on you. [94.7% picked]

Why: Video Discussion

Problem 19:

A rigid two-blade wind turbine that is experiencing zero net torque

(A) has a constant angular velocity, which may be zero. [95.9% picked]

(B) has an angular velocity that is gradually decreasing toward zero. [0.0% picked]

(C) is motionless and may be horizontal or tilted. [2.3% picked]

(D) is motionless and horizontal. [1.9% picked]

Answer: (A) has a constant angular velocity, which may be zero. [95.9% picked]

Why: Video Discussion

Problem 20:

A melon is sitting at equilibrium in the weighing basket of a grocery store spring scale. You lift the melon upward slightly and release it. As the melon bounce up and down, the melon is accelerating downward whenever

(A) it is above equilibrium. [86.8% picked]

(B) its velocity is downward. [1.1% picked]

(C) its velocity is upward. [1.5% picked]

(D) it is below equilibrium. [10.5% picked]

Answer: (A) it is above equilibrium. [86.8% picked]

Why: Video Discussion

Problem 21:

To win a familiar carnival game, you must knock over a stack of three milk jug. The jugs are massive and it's difficult to move them, let alone tip them over. You have your choice of 1-kilogram balls to throw at the jugs. Which of the following is most effective at knocking the jugs over?

(A) A soft ball that is not bouncy. [1.1% picked]

(B) A soft ball that is bouncy. [3.0% picked]

(C) A hard ball that is not bouncy. [4.9% picked]

(D) A hard ball that is bouncy. [91.0% picked]

Answer: (D) A hard ball that is bouncy. [91.0% picked]

Why: Video Discussion

Problem 22:

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

(A) During their collision, the beanbag transfers much less momentum to the much softer trampoline than to the much hard sidewalk. The beanbag's remaining momentum causes the beanbag to bounce upward from the trampoline. [4.9% picked]

(B) 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. [8.6% picked]

(C) During their collision, the beanbag transfers much less energy to the much softer trampoline than to the much harder sidewalk. The beanbag's remaining energy causes the beanbag to bounce upward from the trampoline. [3.0% picked]

(D) When the beanbag collides with the much harder sidewalk, the beanbag receives and wastes nearly all of the collision energy. When the beanbag collides with the much softer trampoline, the trampoline receives and returns nearly all of the collision energy. [83.5% picked]

Answer: (D) When the beanbag collides with the much harder sidewalk, the beanbag receives and wastes nearly all of the collision energy. When the beanbag collides with the much softer trampoline, the trampoline receives and returns nearly all of the collision energy. [83.5% picked]

Why: Video Discussion

Problem 23:

You drop a basketball from rest onto a rigid, motionless granite counter. The ball bounces straight upward and peaks at 50% of its original height. Before you let it drop, the ball had gravitational potential energy due to its height above the counter. What has become of that gravitational potential energy at the moment the ball reaches peak height on its rebound?

(A) 25% is now gravitational potential energy in the ball and 75% is thermal energy in the ball. [0.4% picked]

(B) 50% is now gravitational potential energy in the ball and 50% is kinetic energy in the ball. [12.4% picked]

(C) 50% is now gravitational potential energy in the ball and 50% is thermal energy in the ball. [78.6% picked]

(D) 50% is now gravitational potential energy in the ball and 50% is elastic potential energy in the counter. [8.3% picked]

Answer: (C) 50% is now gravitational potential energy in the ball and 50% is thermal energy in the ball. [78.6% picked]

Why: Video Discussion

Problem 24:

You are pulling your niece uphill on a sled and the sled experiences a small downhill frictional force as it slides uphill on the snow. Your niece is traveling in a straight line at a constant speed. The net force your niece is experiencing is

(A) in the downhill direction. [3.4% picked]

(B) in the uphill direction. [3.0% picked]

(C) zero. [86.8% picked]

(D) in the upward direction. [6.8% picked]

Answer: (C) zero. [86.8% picked]

Why: Video Discussion

Problem 25:

Two children are balanced on a seesaw, but one child weighs twice as much as the other child. The heavier child is sitting half as far from the pivot as is the lighter child. Since the seesaw is balanced, the heavier child is exerting on the seesaw

(A) a torque that is less than the torque the lighter child is exerting. [7.1% picked]

(B) a force that is equal in amount but oppositely directed to the force the lighter child is exerting. [6.8% picked]

(C) a torque that is equal in amount but oppositely directed to the torque the lighter child is exerting. [85.7% picked]

(D) a force that is less than the force the lighter child is exerting. [0.4% picked]

Answer: (C) a torque that is equal in amount but oppositely directed to the torque the lighter child is exerting. [85.7% picked]

Why: Video Discussion

Problem 26:

You are carrying a child on your back as you walk down a hill. The child is traveling straight at a steady speed. In which direction is the force you are exert on the child?

(A) Forward (horizontal) [2.6% picked]

(B) Upward and forward (between vertical and horizontal). [7.9% picked]

(C) Upward (vertical). [82.7% picked]

(D) Downhill (in the direction of your velocity). [6.8% picked]

Answer: (C) Upward (vertical). [82.7% picked]

Why: Video Discussion

Problem 27:

Running on soft dry sand is exhausting, so you switch to running on hard wet sand. The hard wet sand removes less energy from you because

(A) it stops the downward motion of your foot faster and thus absorbs less of your momentum. [24.1% picked]

(B) it pushes up on your foot just as hard as your foot pushes on it, unlike the soft dry sand. [9.0% picked]

(C) it stops the downward motion of your foot faster and thus absorbs more of your momentum. [13.9% picked]

(D) it barely moves downward as you push downward on it, so you do almost zero work on it. [53.0% picked]

Answer: (D) it barely moves downward as you push downward on it, so you do almost zero work on it. [53.0% picked]

Why: Video Discussion

Problem 28:

You are practicing tennis alone by hitting a tennis ball forward toward a cement wall. Each time the ball hits the wall, it bounces backward at high speed so that you can hit it again. During its bounce, the ball

(A) retains approximately all of its momentum but transfers most of its energy to the wall. [6.0% picked]

(B) retains approximately all of its energy but transfers most of its momentum to the wall. [48.5% picked]

(C) retains approximately all of its energy and momentum. [5.3% picked]

(D) retains approximately all of its energy but transfers more forward momentum than it had to the wall. [40.2% picked]

Answer: (D) retains approximately all of its energy but transfers more forward momentum than it had to the wall. [40.2% picked]

Why: Video Discussion

Problem 29:

You jump while standing on a bathroom scale and the scale briefly reads more than your actual weight. During that moment, the scale is exerting an upward force on you that is

(A) equal to your weight and you are accelerating upward. [1.9% picked]

(B) equal to your weight and your velocity is constant. [0.8% picked]

(C) greater than your weight and your velocity is constant. [2.3% picked]

(D) greater than your weight and you are accelerating upward. [95.1% picked]

Answer: (D) greater than your weight and you are accelerating upward. [95.1% picked]

Why: Video Discussion

Problem 30:

Your table at a family-style restaurant has a "lazy Susan" in the middle. This large circular platform rotates frictionlessly so that you can "pass" an entree to your friends by placing it on the platform and then rotating the platform. When the server places a new entree on the platform, it

(A) decreases the platform's angular velocity. [19.9% picked]

(B) makes it harder to change the platform's angular velocity. [76.3% picked]

(C) makes it easier to change the platform's angular velocity. [1.5% picked]

(D) increases the platform's angular velocity. [2.3% picked]

Answer: (B) makes it harder to change the platform's angular velocity. [76.3% picked]

Why: Video Discussion