Midterm Examination 1 Solutions

Problem 1:

You are pushing a shopping cart full of food to the top of a mountain for a party. There are two roads to the top, one twice as long as the other. Each road has a steady slope, although the shorter road is considerably steeper than the longer road. If you take the longer road to the top, [neglect friction or effects due to the air]

(A) your uphill force on the cart will be about the same as the uphill force you would exert while taking the shorter road to the top. [30.6% picked]

(B) the work you do on the cart will be about twice the work you would do while taking the shorter road to the top. [4.1% picked]

(C) the work you do on the cart will be about half the work you would do while taking the shorter road to the top. [7.7% picked]

(D) your uphill force on the cart will be about half the uphill force you would exert while taking the shorter road to the top. [57.7% picked]

Answer: (D) your uphill force on the cart will be about half the uphill force you would exert while taking the shorter road to the top. [57.7% picked]

Why: TBA

Problem 2:

You are pulling a wagon full of children up a hill at constant velocity. Which one of the following statements is true?

(A) The net force on the wagon is zero and you are doing work on the wagon. [73.4% picked]

(B) The net force on the wagon is uphill and you are doing work on the wagon. [6.8% picked]

(C) The net force on the wagon is uphill and you are doing zero work on the wagon. [2.3% picked]

(D) The net force on the wagon is zero and you are doing zero work on the wagon. [17.6% picked]

Answer: (A) The net force on the wagon is zero and you are doing work on the wagon. [73.4% picked]

Why: TBA

Problem 3:

You are enjoying life at a lake when a canoe full of kids rapidly approaches the dock on which you are sitting. You push the canoe backward gently and safely reduce its velocity to zero just before it crashes into the dock. If you had let the canoe crash into the dock, so that the dock suddenly reduced the canoe's velocity to zero, the canoe would have

(A) experienced the same change in momentum, but that momentum change would have taken less time. [91.9% picked]

(B) experienced a smaller change in momentum. [1.4% picked]

(C) experienced a larger change in momentum. [4.5% picked]

(D) experienced the same change in momentum, but that momentum change would have taken more time. [2.3% picked]

Answer: (A) experienced the same change in momentum, but that momentum change would have taken less time. [91.9% picked]

Why: TBA

Problem 4:

The door in front of you is completely free to rotate on its hinges. Only you can affect the door's rotation. You push the door several times in different places and the door's

(A) angular velocity is always proportional to the torque you exert on it. [17.6% picked]

(B) angular acceleration is always proportional to the force you exert on it. [9.0% picked]

(C) angular velocity is always proportional to the force you exert on it. [8.1% picked]

(D) angular acceleration is always proportional to the torque you exert on it. [65.3% picked]

Answer: (D) angular acceleration is always proportional to the torque you exert on it. [65.3% picked]

Why: TBA

Problem 5:

When a person falls onto the ground, that person does not bounce. When the person instead falls onto a trampoline, the person bounces upward. Why does the trampoline allow the person to bounce?

(A) The soft trampoline dents during the collision, stores energy as it dents, and returns that energy to the person during the rebound. [62.6% picked]

(B) The elastic trampoline contains stored force that was put into it by the manufacturer. Some of that stored force is released during the collision and causes the person to bounce. [0.0% picked]

(C) The elastic trampoline contains stored energy that was put into it by the manufacturer. Some of that stored energy is released during the collision and causes the person to bounce. [1.8% picked]

(D) The soft trampoline dents during the collision, stores momentum as it dents, and returns that momentum to the person during the rebound. [35.6% picked]

Answer: (A) The soft trampoline dents during the collision, stores energy as it dents, and returns that energy to the person during the rebound. [62.6% picked]

Why: TBA

Problem 6:

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) velocity. [0.9% picked]

(B) angular momentum. [92.3% picked]

(C) momentum. [3.2% picked]

(D) angular velocity. [3.6% picked]

Answer: (B) angular momentum. [92.3% picked]

Why: TBA

Problem 7:

You take off your shoes to sneak quietly into your room late at night. Unfortunately, it's too dark to see the new 84" flat-screen TV your friend left on the floor and your big toe collides with it. The TV 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 TV than it would have if you had not been wearing socks. [0.5% picked]

(B) less momentum to the TV than it would have if you had not been wearing socks. [7.2% picked]

(C) less energy to the TV than it would have if you had not been wearing socks. [3.2% picked]

(D) the same momentum to the TV, whether or not you are wearing socks, but that transfer involves a smaller force when you are wearing socks. [89.2% picked]

Answer: (D) the same momentum to the TV, whether or not you are wearing socks, but that transfer involves a smaller force when you are wearing socks. [89.2% picked]

Why: TBA

Problem 8:

As each acorn falls from an oak branch located 20 meters above the ground, that acorn takes about 2 seconds to hit the ground. A squirrel on a branch 10 meters above the ground decides to snatch one of those falling acorns as it passes by the squirrel's branch. How long after the acorn starts to fall does the squirrel snatch the passing acorn?

(A) Considerably less than 1 second after the acorn starts to fall. [10.8% picked]

(B) Considerably more than 1 second after the acorn starts to fall. [70.7% picked]

(C) Approximately 0.98 seconds after the acorn starts to fall. [5.4% picked]

(D) 1 second after the acorn starts to fall. [13.1% picked]

Answer: (B) Considerably more than 1 second after the acorn starts to fall. [70.7% picked]

Why: TBA

Problem 9:

As you accelerate forward at the start of a marathon (a long-distance running race), what force or forces is the level ground exerting on you?

(A) A horizontal support force that is directed forward. [2.3% picked]

(B) A horizontal frictional force that is directed forward. [3.6% picked]

(C) An upward support force. [49.5% picked]

(D) An upward support force and a horizontal frictional force that is directed forward. [44.6% picked]

Answer: (D) An upward support force and a horizontal frictional force that is directed forward. [44.6% picked]

Why: TBA

Problem 10:

Which one of the following people is experiencing zero net force?

(A) An acrobat who was tossed straight up like a ball and is now exactly at peak height. [9.0% picked]

(B) A diver who has jumped off the high diving board and is exactly half way to the water. [0.5% picked]

(C) A water-skier who is moving in a straight line at a steady speed behind a boat. [86.0% picked]

(D) An orbiting astronaut who is moving in a circular path around the Earth at 17,000 miles per hour. [4.5% picked]

Answer: (C) A water-skier who is moving in a straight line at a steady speed behind a boat. [86.0% picked]

Why: TBA

Problem 11:

When the batter in a baseball game hits the baseball with the middle part of the baseball bat, the bat vibrates loudly and may even shatter. Why does the ball bounce relatively weakly off the bat in this situation?

(A) Much of the ball's force goes into the bat's vibration and is not available to the rebounding baseball. [1.8% picked]

(B) Much of the collision energy goes into the bat's vibration and is not available to the rebounding baseball. [81.1% picked]

(C) Much of the ball's momentum goes into the bat's vibration and is not available to the rebounding baseball. [15.3% picked]

(D) Much of the ball's angular momentum goes into the bat's vibration and is not available to the rebounding baseball. [1.8% picked]

Answer: (B) Much of the collision energy goes into the bat's vibration and is not available to the rebounding baseball. [81.1% picked]

Why: TBA

Problem 12:

You are walking a dog on a leash when the dog begins chasing a rabbit. As you and the dog move rapidly forward in a straight line at a steady speed, compare the horizontal forces you and the dog exert on the leash that connects you. [Neglect the leash's mass and weight]

(A) The forward force the dog exerts on the leash is greater than the backward force you exert on the leash. [5.0% picked]

(B) The forward force the dog exerts on the leash is equal in amount to the backward force you exert on the leash. [82.0% picked]

(C) Because the leash is not accelerating, each of you exerts zero force on the leash. [12.6% picked]

(D) The backward force you exert on the leash is greater than the forward force the dog exerts on the leash. [0.5% picked]

Answer: (B) The forward force the dog exerts on the leash is equal in amount to the backward force you exert on the leash. [82.0% picked]

Why: TBA

Problem 13:

Your friend throws a lively (bouncy) ball toward you from the other side of a field. To catch that ball, you must let your hands move in the direction of the ball's velocity. If you instead hold your hands motionless as you try to grab the ball, the ball will bounce off your hands and you will drop it. Why does letting your hands move with the ball's motion improve your ability to catch the ball?

(A) Letting your hands move with the ball as the ball pushes on them is required for the ball to do an impulse on your hands, so that it gives up its momentum and doesn't bounce much. [34.8% picked]

(B) If your hands are motionless, their velocity is zero and they exert zero force on the ball. [0.5% picked]

(C) Letting your hands move with the ball as the ball pushes on them is required for the ball to do work on your hands, so that it gives up its energy and doesn't bounce much. [62.0% picked]

(D) If your hands are motionless, their acceleration is zero and they exert zero force on the ball. [2.7% picked]

Answer: (C) Letting your hands move with the ball as the ball pushes on them is required for the ball to do work on your hands, so that it gives up its energy and doesn't bounce much. [62.0% picked]

Why: TBA

Problem 14:

You are exercising at the gym and there is a heavy weight resting motionless on the floor in front of you. You lift that weight upward until it is above your head and you then lower it gently downward to its original, motionless position on the floor. Compare the work you did on the weight while raising it to the work you did while lower it.

(A) You did positive work on the weight while raising it and you did negative that same amount of work on the weight while lower it. [81.5% picked]

(B) You did positive work on the weight in both cases, but you did more work on the weight while raising it than you did while lowering it. [9.5% picked]

(C) You did the same positive work on the weight in both cases. [5.0% picked]

(D) You did positive work on the weight while lowering it and you did negative that same amount of work on the weight while raising it. [4.1% picked]

Answer: (A) You did positive work on the weight while raising it and you did negative that same amount of work on the weight while lower it. [81.5% picked]

Why: TBA

Problem 15:

A car heading northward on a road skids to a stop to avoid hitting something in the road. As the car skids to a stop, what work is done? [Neglect effects due to the air and any movement of the road itself]

(A) The road does positive work on the car and the car does positive work on the road. [1.4% picked]

(B) The road does negative work on the car, but the car does positive work on the road. [8.1% picked]

(C) The road does positive work on the car, but the car does negative work on the road. [5.4% picked]

(D) The road does negative work on the car, but the car does zero work on the road. [85.1% picked]

Answer: (D) The road does negative work on the car, but the car does zero work on the road. [85.1% picked]

Why: TBA

Problem 16:

You are acting in a movie and the script calls for you to weigh yourself on a spring scale. Being a vain actor, you want the scale to read less than your weight, so you film the scene in an elevator. For the scale to read less than your weight as you weigh yourself in that elevator, the elevator must be

(A) accelerating upward. [11.3% picked]

(B) accelerating downward. [82.9% picked]

(C) moving downward at constant velocity. [5.4% picked]

(D) moving upward at constant velocity. [0.5% picked]

Answer: (B) accelerating downward. [82.9% picked]

Why: TBA

Problem 17:

It is difficult to cut thick cardboard with a pair of scissors. The farther the cardboard is from the scissors' pivot, the harder it becomes for you to rotate the scissors closed because the cardboard then

(A) exerts larger forces on the scissors and keeps them from accelerating. [19.8% picked]

(B) has more momentum to stop the scissors from moving and to prevent them from cutting through the cardboard. [5.9% picked]

(C) exerts larger forces on the scissors and causes them to accelerate. [0.5% picked]

(D) has longer lever arms with which to produce larger torques on the scissors to oppose the scissors rotating closed. [73.9% picked]

Answer: (D) has longer lever arms with which to produce larger torques on the scissors to oppose the scissors rotating closed. [73.9% picked]

Why: TBA

Problem 18:

You are stretching a rubber band by holding its left end motionless with your left hand while you move its right end to the right with your right hand. The rubber band's energy increases as it stretches, so where is that energy coming from?

(A) Your right hand is transferring energy to the rubber band as you stretch it. [88.7% picked]

(B) Both of your hands are transferring energy to the rubber band as you stretch it. [7.2% picked]

(C) Your left hand is transferring energy to the rubber band as you stretch it. [1.4% picked]

(D) The rubber band is creating that energy as you stretch it. [2.7% picked]

Answer: (A) Your right hand is transferring energy to the rubber band as you stretch it. [88.7% picked]

Why: TBA

Problem 19:

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 girl jumps mostly upward while the boy jumps mostly forward. After they leave the dock,

(A) the boy reaches the water before the girl. [86.0% picked]

(B) the two children reach the water at the same moment and but the boy travels farther from the dock than does the girl. [9.9% picked]

(C) the two children reach the water at the same moment and at the same distance from the dock. [0.9% picked]

(D) the girl reaches the water before the boy. [3.2% picked]

Answer: (A) the boy reaches the water before the girl. [86.0% picked]

Why: TBA

Problem 20:

You are a firefighter and the alarm sounds in your fire station. As you slide down the fire pole at constant velocity, your gravitational potential energy is decreasing. What is becoming of that energy?

(A) It is becoming thermal energy. [64.0% picked]

(B) It is becoming downward momentum. [1.4% picked]

(C) It is becoming kinetic energy. [34.2% picked]

(D) It is becoming downward force. [0.5% picked]

Answer: (A) It is becoming thermal energy. [64.0% picked]

Why: TBA

Problem 21:

You are pushing a table across the floor. As the table moves forward at constant velocity, compare the horizontal force you exert on the table to the horizontal frictional force the floor exerts on the table. [Neglect any effects due to the air]

(A) Your horizontal force on the table is greater in amount than the floor's horizontal frictional force on the table. [5.9% picked]

(B) Your horizontal force on the table is less in amount than the floor's horizontal frictional force on the table. [0.0% picked]

(C) Your horizontal force on the table is equal to the floor's horizontal frictional force on the table. [8.1% picked]

(D) Your horizontal force on the table is equal in amount but opposite in direction to the floor's horizontal frictional force on the table. [86.0% picked]

Answer: (D) Your horizontal force on the table is equal in amount but opposite in direction to the floor's horizontal frictional force on the table. [86.0% picked]

Why: TBA

Problem 22:

It has been a long day and you casually throw your backpack onto your bed. The backpack is in free fall prior to touching the bed and is moving downward. When during its initial landing on the bed does the backpack reach its maximum downward speed?

(A) When it reaches its lowest point during the landing and is denting the bed as deeply downward as it ever will. [6.8% picked]

(B) When it reaches equilibrium, where the bed is exactly supporting its weight for the first time. [74.8% picked]

(C) When it is about to touch the bed for the first time and the backpack and bed are not yet in contact. [13.5% picked]

(D) When it has just touched the bed for the first time and the backpack and bed are now in contact. [5.0% picked]

Answer: (B) When it reaches equilibrium, where the bed is exactly supporting its weight for the first time. [74.8% picked]

Why: TBA

Problem 23:

A dozen children are riding the painted wooden horses on a carousel at the fair. They all sit on horses at the outer edge of the carousel disk, where they can see and be seen. If those children had instead sat on horses near the center of the carousel disk, how would that have changed the carousel's behavior?

(A) The carousel would have had less rotational mass and would have undergone angular acceleration less easily. [5.4% picked]

(B) The carousel would have had more rotational mass and would have undergone angular acceleration less easily. [2.3% picked]

(C) The carousel would have had less rotational mass and would have undergone angular acceleration more easily. [82.4% picked]

(D) The carousel would have had more rotational mass and would have undergone angular acceleration more easily. [9.9% picked]

Answer: (C) The carousel would have had less rotational mass and would have undergone angular acceleration more easily. [82.4% picked]

Why: TBA

Problem 24:

You are playing soccer on an ordinary level soccer field. You kick the ball and it travels high above the ground before landing in the goal. Nothing touches the ball as it arcs from your foot to the goal, so when during that arc does the ball have the slowest speed (the smallest amount of velocity)? [Neglect any effects due to the air]

(A) It travels slowest just before it hits the ground in the goal. [1.4% picked]

(B) It travels slowest just after it leaves your foot. [3.2% picked]

(C) It travels slowest when it reaches peak height above the ground. [73.4% picked]

(D) Because of inertia, its speed is constant throughout the arc. [22.1% picked]

Answer: (C) It travels slowest when it reaches peak height above the ground. [73.4% picked]

Why: TBA

Problem 25:

A plastic step used in an exercise class consists of a plastic board supported at each end by a plastic block. When you step on the board and stand motionless, the board bends downward 1 millimeter. Your friend weighs twice as much as you do. When your friend steps on the board and stands motionless, the board bends downward

(A) 4 millimeters [4.1% picked]

(B) 2 millimeters [95.5% picked]

(C) 1 millimeter [0.0% picked]

(D) 8 millimeters [0.5% picked]

Answer: (B) 2 millimeters [95.5% picked]

Why: TBA

Problem 26:

You place a stiff ruler on a pencil and then stack coins on the two ends of the ruler until it is balanced. The ruler and its coins are then able to remain motionless on the pencil pivot. What did you have to do to achieve this balanced condition?

(A) The overall center of gravity of the ruler and its coins had to be directly above the pencil pivot. [37.7% picked]

(B) The forces that the coins on each side of the ruler exerted on the ruler had to be equal. [20.9% picked]

(C) The weights of the coins on each side of the ruler had to be equal. [12.7% picked]

(D) The overall center of gravity of the ruler and its coins had to coincide with the overall center of mass of the ruler and its coins. [28.6% picked]

Answer: (A) The overall center of gravity of the ruler and its coins had to be directly above the pencil pivot. [37.7% picked]

Why: TBA

Problem 27:

You toss a basketball straight up. Disregarding any effects of due to the air, what force or forces are acting on the basketball while it is above your hands?

(A) Its weight. [92.3% picked]

(B) Its weight along with a steadily decreasing upward force. [0.5% picked]

(C) A steadily decreasing upward force from the moment it leaves your hands until it reaches its highest point and then a steadily increasing downward force as the basketball returns toward your hands. [3.6% picked]

(D) Its weight along with an upward force that steadily decreases until the basketball reaches its highest point. After that point, there is only the constant downward force of gravity. [3.6% picked]

Answer: (A) Its weight. [92.3% picked]

Why: TBA

Problem 28:

If you drop a baseball and a bowling ball from rest at the same time and from the same height, how will they move? [Neglect any effects due to the air]

(A) The bowling ball has the greater weight, so it will accelerate downward more rapidly than the baseball and it will hit the ground first. [0.5% picked]

(B) They will accelerate downward at the same rate and hit the ground at the same time. [87.4% picked]

(C) The baseball has the smaller mass, so it will accelerate downward more rapidly than the bowling ball and it will hit the ground first. [0.9% picked]

(D) They will hit the ground at the same time even though the bowling ball experienced a greater downward acceleration than the baseball. [11.3% picked]

Answer: (B) They will accelerate downward at the same rate and hit the ground at the same time. [87.4% picked]

Why: TBA

Problem 29:

Your new yacht (new boat) is being launched and is moving down a ramp at constant velocity, on its way to the water. You are standing on the yacht, waving at your admiring friends. In which direction is the yacht pushing on you?

(A) Horizontally in the forward direction [1.4% picked]

(B) Uphill (along the ramp in the direction away from the water) [2.3% picked]

(C) Downhill (along the ramp in the direction toward the water) [8.6% picked]

(D) Upward [87.8% picked]

Answer: (D) Upward [87.8% picked]

Why: TBA

Problem 30:

You are riding your motorcycle off a building's horizontal roof and expect to land on the level parking lot below. Unfortunately, you fall asleep just as the motorcycle leaves the roof and immediately let go of the motorcycle. The motorcycle is much heavier than you. When and where do the two of you land on the ground? [Neglect any effects due to the air]

(A) The motorcycle lands considerably earlier in time and closer to the building than you do. [1.8% picked]

(B) You both land at the same time, but the motorcycle lands significantly farther from the building than you do. [21.2% picked]

(C) You both land at the same distance from the building, but the motorcycle lands significantly earlier in time than you do. [2.7% picked]

(D) You both land at the same time and at the same distance from the building. [74.3% picked]

Answer: (D) You both land at the same time and at the same distance from the building. [74.3% picked]

Why: TBA