Midterm Examination 1 Solutions

Problem 1:

A car traveling at 60 mph veers off the road and hits a tree. The car immediately comes to a complete stop. Fortunately, the airbag inflates and the driver comes to a stop in the airbag instead of coming to a stop on the steering wheel. Hitting the airbag rather than the steering wheel saves the driver's life because the driver

(A) carries less force with her before colliding with the airbag than she would have carried with her if there were no airbag. [0.4% picked]

(B) transfers more momentum to the airbag than she would have transferred to the steering wheel. [1.3% picked]

(C) transfers all of her momentum to whatever stops her, but that transfer is slower and involves a smaller force when she hits the airbag. [97.4% picked]

(D) transfers less momentum to the airbag than she would have transferred to the steering wheel. [0.9% picked]

Answer: (C) transfers all of her momentum to whatever stops her, but that transfer is slower and involves a smaller force when she hits the airbag. [97.4% picked]

Why: Video Discussion

Problem 2:

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 upward whenever

(A) its velocity is upward. [2.6% picked]

(B) its velocity is downward. [5.6% picked]

(C) it is below equilibrium. [80.1% picked]

(D) it is above equilibrium. [11.7% picked]

Answer: (C) it is below equilibrium. [80.1% picked]

Why: Video Discussion

Problem 3:

You have landed a job assembling cheap furniture for a budget hotel chain. Unfortunately, the pieces don't fit together well and you have to pound them together with a 1-kg mallet. You want the 1-kg mallet that exerts the most force on the furniture, so as to overwhelm any resisting forces in the ill-fitting pieces, and that transfers the most momentum to those pieces, so as to complete the assembly with the fewest mallet blows. To achieve these goals, the part of the mallet that hits the furniture should be

(A) soft and dead. [2.2% picked]

(B) stiff and dead. [12.1% picked]

(C) stiff and bouncy. [81.4% picked]

(D) soft and bouncy. [4.3% picked]

Answer: (C) stiff and bouncy. [81.4% picked]

Why: Video Discussion

Problem 4:

Alice and Bob are carrying 50 pound bags of seeds from the sidewalk to your penthouse garden, 100 feet above the sidewalk. Alice carries bags up the stairs. Bob carries bags up a vertical ladder. Compare the work each person does on a bag while moving it from the sidewalk to the garden. [Neglect friction and air resistance, and assume that bags are motionless at the start and finish.]

(A) Bob does more work on a bag. [3.5% picked]

(B) Alice and Bob do the same work on a bag. [94.4% picked]

(C) Alice does more work on a bag. [1.7% picked]

(D) The person who completed the task first does more work on a bag. [0.4% picked]

Answer: (B) Alice and Bob do the same work on a bag. [94.4% picked]

Why: Video Discussion

Problem 5:

You make a sharp left turn in your car and your cell phone slips off the dashboard and out the right passenger window. The cell phone exited the car because

(A) the car did not exert enough leftward force on the phone to make the phone accelerate with the car. [48.1% picked]

(B) the phone's weight pulled the phone out the window. [23.4% picked]

(C) the car exerted a rightward centrifugal force on the phone that pushed the phone out the window. [27.3% picked]

(D) the dashboard of the turning car tilted sharply to become a ramp and the downhill ramp force pushed the phone out the window. [1.3% picked]

Answer: (A) the car did not exert enough leftward force on the phone to make the phone accelerate with the car. [48.1% picked]

Why: Video Discussion

Problem 6:

An acrobatic bicyclist rides a ramp and zooms through the sky for several seconds. He does a few amazing stunts on the bicycle before landing back on the ground. While he and his bicycle are not touching the ground and neglecting any effects due to the air, the one aspect of their motion that is constant is their total

(A) velocity. [3.9% picked]

(B) angular momentum. [78.4% picked]

(C) angular velocity. [5.6% picked]

(D) momentum. [11.7% picked]

Answer: (B) angular momentum. [78.4% picked]

Why: Video Discussion

Problem 7:

The plastic step you use in an exercise class bends downward 1.0 centimeter when you stand motionless on its top. When a person twice your weight stands on that same step, it bends downward approximately

(A) 1.0 centimeter. [0.9% picked]

(B) 2.0 centimeters. [93.1% picked]

(C) 1.4 centimeters. [1.7% picked]

(D) 4.0 centimeters. [4.3% picked]

Answer: (B) 2.0 centimeters. [93.1% picked]

Why: Video Discussion

Problem 8:

You're driving along a level road and approach a red light. You press the brake pedal and the car slows to a stop. As you stop, what happens to the car's forward momentum?

(A) It becomes thermal momentum in the brakes and brake pads. [31.6% picked]

(B) It is transferred to the ground by way of friction forces. [62.3% picked]

(C) It becomes potential momentum in the entire car. [0.9% picked]

(D) It is transferred to the ground by way of support forces. [5.2% picked]

Answer: (B) It is transferred to the ground by way of friction forces. [62.3% picked]

Why: Video Discussion

Problem 9:

When a box skids to a stop on a motionless table, what work is done by those two objects?

(A) The box does positive work on the table. [3.5% picked]

(B) Both objects do negative work on one another. [7.4% picked]

(C) The table does negative work on the box. [74.0% picked]

(D) The box does positive work on the table, the table does negative work on the box. [14.3% picked]

Answer: (C) The table does negative work on the box. [74.0% picked]

Why: Video Discussion

Problem 10:

You are watching a tennis match and the server has just hit the ball toward her opponent on the other side of the court. Neglect any effects due to the air. Once the tennis ball has left the server's tennis racket and is traveling forward, the ball experiences

(A) a forward horizontal force that remains constant all the way to the opponent's side of the court. [4.8% picked]

(B) a forward horizontal force that diminishes gradually as the ball approaches the opponent's side of the court. [3.5% picked]

(C) a forward horizontal force until it reaches the midpoint of its trip across the tennis court and then a backward horizontal force for the remainder of its trip. [0.9% picked]

(D) no horizontal force in the forward direction. [90.9% picked]

Answer: (D) no horizontal force in the forward direction. [90.9% picked]

Why: Video Discussion

Problem 11:

Your favorite marble rolls off the deck and falls for 2 seconds before hitting the ground. After only 1 second of falling, the marble was

(A) closer to the ground than to the deck. [6.9% picked]

(B) halfway between the deck and the ground. [6.5% picked]

(C) much closer to the deck than to the ground. [74.0% picked]

(D) a few inches closer to the deck than to the ground. [12.6% picked]

Answer: (C) much closer to the deck than to the ground. [74.0% picked]

Why: Video Discussion

Problem 12:

Two children with different weights are riding on a seesaw. Why does having the lighter child sit farther from the pivot allow that child to balance the heavier child so that seesaw rotates properly?

(A) The farther a child sits from the pivot, the more momentum that child's weight transfers to the seesaw. [7.8% picked]

(B) The farther a child sits from the pivot, the more torque that child's weight produces on the seesaw. [88.3% picked]

(C) Sitting farther from the pivot increases the child's weight so that the child exerts more torque on the seesaw. [2.6% picked]

(D) Sitting farther from the pivot increases the child's weight so that the child exerts more force on the seesaw. [1.3% picked]

Answer: (B) The farther a child sits from the pivot, the more torque that child's weight produces on the seesaw. [88.3% picked]

Why: Video Discussion

Problem 13:

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) greater than your weight and your velocity is constant. [2.6% picked]

(B) greater than your weight and you are accelerating upward. [91.8% picked]

(C) equal to your weight and you are accelerating upward. [5.2% picked]

(D) equal to your weight and your velocity is constant. [0.4% picked]

Answer: (B) greater than your weight and you are accelerating upward. [91.8% picked]

Why: Video Discussion

Problem 14:

Your friend is sitting still on a playground merry-go-round and it is spinning at a steady pace (neglect friction and air resistance). You give a motionless bowling ball to your friend so that the ball suddenly starts spinning with the merry-go-round. As the ball begins to move with the merry-go-round, the merry-go-round's

(A) angular velocity increases. [7.8% picked]

(B) angular momentum increases. [13.0% picked]

(C) angular momentum decreases. [14.7% picked]

(D) angular velocity decreases. [64.5% picked]

Answer: (D) angular velocity decreases. [64.5% picked]

Why: Video Discussion

Problem 15:

You are riding a rollercoaster with a loop-the-loop. As you pass through the top of that loop and are upside down, you remain pressed into your seat. You are accelerating

(A) upward at a rate equal in amount to the acceleration due to gravity. [35.5% picked]

(B) downward at the acceleration due to gravity. [29.9% picked]

(C) downward at more than the acceleration due to gravity. [26.8% picked]

(D) downward at less than the acceleration due to gravity. [7.4% picked]

Answer: (C) downward at more than the acceleration due to gravity. [26.8% picked]

Why: Video Discussion

Problem 16:

You are walking at constant velocity along a horizontal sidewalk, wearing a heavy backpack. You are doing

(A) (positive) work on the backpack and it is doing negative work on you. [15.6% picked]

(B) zero work on the backpack and it is doing zero work on you. [80.1% picked]

(C) negative work on the backpack and it is doing (positive) work on you. [1.7% picked]

(D) (positive) work on the backpack and it is doing (positive) work on you. [2.6% picked]

Answer: (B) zero work on the backpack and it is doing zero work on you. [80.1% picked]

Why: Video Discussion

Problem 17:

You and your friends are having a snowball fight. There are two identical snowballs approaching you, one traveling at 2 feet per second and one traveling 10 feet per second. You wisely try to avoid the faster snowball because it is carrying

(A) 5 times as much kinetic energy as the slower snowball. [16.5% picked]

(B) 8 times as much kinetic energy as the slower snowball. [3.0% picked]

(C) 64 times as much kinetic energy as the slower snowball. [12.1% picked]

(D) 25 times as much kinetic energy as the slower snowball. [68.4% picked]

Answer: (D) 25 times as much kinetic energy as the slower snowball. [68.4% picked]

Why: Video Discussion

Problem 18:

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 decreases. [69.7% picked]

(B) It remains constant because no torque acts on the disk. [8.7% picked]

(C) It increases. [16.5% picked]

(D) It remains constant because angular velocity is conserved. [5.2% picked]

Answer: (A) It decreases. [69.7% picked]

Why: Video Discussion

Problem 19:

To win a big stuffed animal at the local fair, you must toss a bouncy ball so that it comes to rest in a shallow ceramic bowl. But each time your ball lands on the bowl, the ball bounces and falls to the floor. This game is difficult to win because when the ball hits the bowl, the ball

(A) transfers much energy but almost zero momentum to the bowl. [5.6% picked]

(B) almost zero momentum and almost zero energy to the bowl. [4.3% picked]

(C) much momentum and much energy to the bowl. [1.7% picked]

(D) transfers much momentum but almost zero energy to the bowl. [88.3% picked]

Answer: (D) transfers much momentum but almost zero energy to the bowl. [88.3% picked]

Why: Video Discussion

Problem 20:

You are riding cable car from a valley to a mountaintop and are traveling toward the mountaintop at a steady pace. In which direction is the cable car pushing you?

(A) Directly upward. [78.8% picked]

(B) Directly toward the valley. [2.2% picked]

(C) Directly toward the mountaintop. [4.8% picked]

(D) Up and forward, toward a point somewhat above the mountaintop. [14.3% picked]

Answer: (A) Directly upward. [78.8% picked]

Why: Video Discussion

Problem 21:

Your Frisbee is caught in a tree, so you have just thrown a small pebble and a large rock at the Frisbee. The two stones are halfway to their target and now have the same speed and the same direction of travel. Why should you expect the large rock to be more effective at knocking the Frisbee out of the tree than the small pebble?

(A) The large rock is carrying more momentum than the small pebble. [75.8% picked]

(B) The large rock is carrying more velocity than the small pebble. [0.4% picked]

(C) The large rock is carrying more force than the small pebble. [23.8% picked]

(D) The large rock is carrying more acceleration than the small pebble. [0.0% picked]

Answer: (A) The large rock is carrying more momentum than the small pebble. [75.8% picked]

Why: Video Discussion

Problem 22:

You are rollerblading on ramps at the skate park. As you try steeper and steeper ramps, you find that

(A) your velocity becomes greater. [10.0% picked]

(B) your velocity is the same as on a shallower ramp. [0.9% picked]

(C) your speed is smaller when you are heading up the ramp and greater when you are moving down the ramp. [51.1% picked]

(D) your acceleration becomes greater. [37.7% picked]

Answer: (D) your acceleration becomes greater. [37.7% picked]

Why: Video Discussion

Problem 23:

You jump off a porch onto a trampoline and bounce nicely. During your trip, you accelerate downward until the moment when you

(A) are about to touch the trampoline. [1.3% picked]

(B) reach the bottom of your bounce (you dent the trampoline as deeply as possible). [10.4% picked]

(C) reach equilibrium in the trampoline (you dent the trampoline enough that it supports your weight). [82.7% picked]

(D) touch the trampoline. [5.6% picked]

Answer: (C) reach equilibrium in the trampoline (you dent the trampoline enough that it supports your weight). [82.7% picked]

Why: Video Discussion

Problem 24:

A flat-bed truck is trying to deliver a huge load of ice. The driver is in a hurry and steps hard on the accelerator. As the truck leaps forward, the ice slips off the back of the truck bed and falls to the ground. What caused the ice to leave the truck's bed?

(A) The truck bed exerted a static friction force on the ice, pushing the ice backward so that it fell off the truck. [0.4% picked]

(B) The truck bed exerted a sliding friction force on the ice, pushing the ice backward so that it fell off the truck. [1.3% picked]

(C) The ice's inertia kept it at approximately constant velocity as the truck accelerated out from under the ice. [97.4% picked]

(D) The truck bed exerted a support force on the ice, pushing the ice backward so that it fell off the truck. [0.9% picked]

Answer: (C) The ice's inertia kept it at approximately constant velocity as the truck accelerated out from under the ice. [97.4% picked]

Why: Video Discussion

Problem 25:

You are involved in a game of tug-o-war with a plastic clothesline. The two teams are pulling at opposite ends of the cord and each team is trying to drag the other team across a line marked on the ground. After a few minutes without progress, your team suddenly pulls the rope toward you especially hard. The opposing team has anticipated this action and is able to keep their end of the rope from moving. Your end of the rope stretches toward you and the rope breaks. Breaking the rope required energy and that energy was provided by

(A) your team. [87.9% picked]

(B) the opposing team. [1.7% picked]

(C) neither team. It was instead provided by chemical potential energy in the rope itself. [0.9% picked]

(D) both teams. [9.5% picked]

Answer: (A) your team. [87.9% picked]

Why: Video Discussion

Problem 26:

You are pushing your desk across the floor in a straight line at a steady speed. 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 force that friction is exerting on it. [7.8% picked]

(B) The amount of force that you're exerting on the desk must be more than the amount of its weight. [1.3% picked]

(C) If you were to exert twice as much force on the desk, it would slide across the floor twice as fast. [1.3% picked]

(D) The amount of force that you're exerting on the desk must be equal to the amount of force that friction is exerting on it. [89.6% picked]

Answer: (D) The amount of force that you're exerting on the desk must be equal to the amount of force that friction is exerting on it. [89.6% picked]

Why: Video Discussion

Problem 27:

You have two baseball bats: one a conventional wooden bat and the other a high-tech aluminum bat. The two bats have the same shape and mass, but the aluminum bat has a softer and bouncier surface. When the two bats are swung equally fast at the same pitched ball, the aluminum bat hits the ball farther than the wooden bat because the aluminum bat

(A) exerts more force on the ball than the ball exerts on the bat, so that those two forces don't cancel and the ball accelerates forward. [2.2% picked]

(B) receives a larger fraction of the collision energy and returns almost all of that energy as rebound energy. [82.3% picked]

(C) prolongs the collision between the bat and ball so that the ball experiences a greater forward force. [13.4% picked]

(D) carries more momentum than the wooden bat and transfers all of that momentum to the ball. [1.7% picked]

Answer: (B) receives a larger fraction of the collision energy and returns almost all of that energy as rebound energy. [82.3% picked]

Why: Video Discussion

Problem 28:

You are riding a carousel at the fair. Suddenly, your horse breaks free from the carousel. Neglecting gravity, in which direction do you travel?

(A) You travel outward, directly away from center of the carousel. [12.1% picked]

(B) You travel inward, directly toward the center of the carousel. [9.5% picked]

(C) You continue straight ahead, in the direction of your velocity before the horse broke free. [61.9% picked]

(D) You travel forward and outward, in a direction that lies between straight ahead and directly away from the center of the carousel. [16.5% picked]

Answer: (C) You continue straight ahead, in the direction of your velocity before the horse broke free. [61.9% picked]

Why: Video Discussion

Problem 29:

Two toy cars roll off a level table side-by-side at the same time and in the same direction The cars soon hit the level floor below the table. The red car weighs twice as much as the blue car, but the blue car left the table at twice the horizontal speed of the red car. In this situation,

(A) the red car hits the floor much sooner than the blue car, but the blue car hits considerably farther from the table than the red car. [3.5% picked]

(B) the two cars hit the floor at approximately the same distance from the table, but the red car hits much sooner than the blue car. [1.3% picked]

(C) the blue car hits the floor much sooner than the red car, but the blue car hits considerably farther from the table than the red car. [3.9% picked]

(D) the two cars hit the floor at approximately the same time, but the blue car hits considerably farther from the table than the red car. [91.3% picked]

Answer: (D) the two cars hit the floor at approximately the same time, but the blue car hits considerably farther from the table than the red car. [91.3% picked]

Why: Video Discussion

Problem 30:

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 but the child who jumps forward travels farther from the dock than does the other child. [17.7% picked]

(B) the two children reach the water at the same moment and at the same distance from the dock. [0.4% picked]

(C) the child who jumps forward reaches the water before the child who jumps upward. [81.4% picked]

(D) the child who jumps upward reaches the water before the child who jumps forward. [0.4% picked]

Answer: (C) the child who jumps forward reaches the water before the child who jumps upward. [81.4% picked]

Why: Video Discussion