Midterm Examination 1 Solutions

Problem 1:

You accidentally leave a cup of coffee on the horizontal roof of your car before driving on the level roads of your city. That cup is most likely to fall of the roof when your car is [neglect effects due to the air]

(A) moving backward at constant velocity. [0.4% picked]

(B) moving forward as fast as possible in a straight line at a steady speed. [3.0% picked]

(C) speeding up or slowing down, but not when it is turning. [2.6% picked]

(D) speeding up, slowing down, or turning. [94.0% picked]

Answer: (D) speeding up, slowing down, or turning. [94.0% picked]

Why: TBA

Problem 2:

The ball that you throw to your friend travels in an arc, first rising to peak height and then descending. While the ball isn't touching anything and neglecting effects due to the air, when does the ball have the largest acceleration?

(A) Its acceleration is constant throughout the arc. [58.4% picked]

(B) Its acceleration is largest when it is descending. [28.3% picked]

(C) Its acceleration is largest when it is rising. [6.4% picked]

(D) Its acceleration is largest when it is at peak height. [6.9% picked]

Answer: (A) Its acceleration is constant throughout the arc. [58.4% picked]

Why: TBA

Problem 3:

You are standing in the middle of a bus that is moving forward at constant velocity. You accidentally drop a bottle of salad oil on the floor and it breaks. Suddenly, the oily floor cannot exert any frictional forces on your feet. Nothing else is touching you, so you

(A) shift toward the side of the bus (perpendicular to the direction of its velocity). [0.0% picked]

(B) remain in the middle of the bus. [81.1% picked]

(C) shift toward the front of the bus (in the direction of its velocity). [2.1% picked]

(D) shift toward the back of the bus (opposite its velocity). [16.7% picked]

Answer: (B) remain in the middle of the bus. [81.1% picked]

Why: TBA

Problem 4:

Your restaurant table has a Lazy Susan, a freely rotating circular platform used to move food dishes around the table. The empty platform responds easily to a gentle torque. How does the platform respond to that same torque once there are food dishes on the platform?

(A) The dishes increase the platform's angular momentum, so it has more force. [3.4% picked]

(B) The dishes increase the platform's energy, so it resists acceleration and center of mass. [0.9% picked]

(C) The platform's angular velocity is smaller because the dishes increase its rotational mass. [33.0% picked]

(D) The platform's angular acceleration is smaller because the dishes increase its rotational mass. [62.7% picked]

Answer: (D) The platform's angular acceleration is smaller because the dishes increase its rotational mass. [62.7% picked]

Why: TBA

Problem 5:

A building has two balconies, one twice as high above you as the other. When a ball is dropped from the lower balcony reaches your hand, it is traveling 10 meters-per-second (about 22 miles-per-hour). When that ball is dropped from the upper balcony and falls twice as far, how fast is it traveling when it reaches your hand? [neglect any effects due to the air.]

(A) More than 10 meters-per-second, but less than 20 meters-per-second. [17.6% picked]

(B) 20 meters-per-second. [17.2% picked]

(C) 10-meters-per-second. [6.4% picked]

(D) More than 20 meters-per-second. [58.8% picked]

Answer: (A) More than 10 meters-per-second, but less than 20 meters-per-second. [17.6% picked]

Why: TBA

Problem 6:

A sheet of paper is lying flat on a horizontal table. There are several coins sitting motionless on the paper. You pull the paper suddenly toward the right and the coins barely move. The coins remain almost in place because

(A) the force of each coin's momentum pushes it oppose the frictional force of the paper and those two forces cancel. [6.4% picked]

(B) the sliding frictional forces that the paper exerts on the coins are too weak to cause them to accelerate. [38.6% picked]

(C) the static frictional forces that the paper exerts on the coins are too weak to cause them to accelerate. [12.4% picked]

(D) there is too little time for the paper's frictional forces to transfer significant rightward momentum to the coins. [42.5% picked]

Answer: (D) there is too little time for the paper's frictional forces to transfer significant rightward momentum to the coins. [42.5% picked]

Why: TBA

Problem 7:

To win a huge stuffed bear at the county fair, all you have to do is toss a US quarter coin toward a smooth immovable glass plate and have the quarter remain on the plate. This task is surprisingly difficult because the coins keep bouncing off the plate and onto the ground. What prevents the coin from landing on the plate and stopping completely?

(A) The coin isn't massive enough to remain on the plate. [3.4% picked]

(B) The coin isn't heavy enough to remain on the plate. [3.9% picked]

(C) It can't do an impulse on the plate. [5.6% picked]

(D) It can't do work on the plate or waste significant energy via sliding friction. [87.1% picked]

Answer: (D) It can't do work on the plate or waste significant energy via sliding friction. [87.1% picked]

Why: TBA

Problem 8:

You get on your bicycle near the top of a long, smooth hill. After a moment of motionlessness, you start rolling downhill faster and faster. You did not use the pedals, so what caused the bicycle wheels to start turning?

(A) The bicycle exerted downhill forces on the centers of the bicycle wheels. [2.1% picked]

(B) The bicycle exerted downward (vertical) forces on the centers of the bicycle wheels. [8.2% picked]

(C) The ground exerted uphill frictional forces on the bottoms of the bicycle wheels. [63.5% picked]

(D) The ground exerted downhill frictional forces on the bottoms of the bicycle wheels. [26.2% picked]

Answer: (C) The ground exerted uphill frictional forces on the bottoms of the bicycle wheels. [63.5% picked]

Why: TBA

Problem 9:

You are riding a store's escalator and are currently moving from the first floor to the second floor at a constant velocity. In which direction is the escalator pushing on you?

(A) Uphill (toward the escalator's second floor exit). [9.0% picked]

(B) Forward (horizontally). [3.0% picked]

(C) Somewhat above uphill (toward a point above the escalator's second floor exit). [4.7% picked]

(D) Upward (vertically). [83.3% picked]

Answer: (D) Upward (vertically). [83.3% picked]

Why: TBA

Problem 10:

You are swinging back and forth on a swing at the local playground. As you swing forward and pass directly below the supporting pivot, you feel heavier than normal. Your increased apparent weight is due to

(A) your force being forward, so that you feel both your normal weight and that additional force. [12.9% picked]

(B) your acceleration being upward, so that you have a downward feeling of acceleration. [65.2% picked]

(C) your mass increasing with your speed, so that you have an enhanced weight. [1.7% picked]

(D) your velocity being downward, so that you have downward inertial weight. [20.2% picked]

Answer: (B) your acceleration being upward, so that you have a downward feeling of acceleration. [65.2% picked]

Why: TBA

Problem 11:

A basketball basket is mounted on a rigid, immovable backboard. A player can bounce the basketball off the backboard on its way to the basket. When the basketball hits the backboard and rebounds, what conserved physical quantities does the basketball transfer to the backboard?

(A) Both energy and momentum. [20.7% picked]

(B) Force but not energy or momentum. [3.0% picked]

(C) Energy but not momentum. [11.2% picked]

(D) Momentum but not energy. [65.1% picked]

Answer: (D) Momentum but not energy. [65.1% picked]

Why: TBA

Problem 12:

You board a motionless bus at a bus stop. You stand on your skateboard in the middle of the bus. As the bus accelerates forward, you find yourself shifting toward the back of the bus. From the perspective of a person standing motionless at the bus stop, what is your velocity as you shift?

(A) Your velocity is forward and equal in amount to half the velocity of the bus. [5.6% picked]

(B) Your velocity is backward and equal in amount to half the velocity of the bus. [2.2% picked]

(C) Your velocity is zero. [58.6% picked]

(D) Your velocity is backward and equal in amount to the velocity of the bus. [33.6% picked]

Answer: (C) Your velocity is zero. [58.6% picked]

Why: TBA

Problem 13:

The ball that you throw to your friend travels in an arc, first rising to peak height and then descending. While the ball isn't touching anything and neglecting effects due to the air, when does the ball have the smallest speed?

(A) Its speed is smallest when it is descending. [1.3% picked]

(B) Its speed is smallest when it is rising. [1.7% picked]

(C) Its speed is constant throughout the arc. [22.7% picked]

(D) Its speed is smallest when it is at peak height. [74.2% picked]

Answer: (D) Its speed is smallest when it is at peak height. [74.2% picked]

Why: TBA

Problem 14:

You are in a motionless elevator at the top floor of a tall building. There is a spring scale under your feet, currently correctly reporting your weight. You press a button and the elevator starts downward, moves steadily through the middle floors, and soon stops at the ground floor. What values does the scale report during that trip?

(A) It reports your weight throughout the trip. [4.7% picked]

(B) It reports less than your weight as you start downward, then your weight as you coast through the middle floors, then more than your weight as you slow to a stop. [77.3% picked]

(C) It reports less than your weight throughout the trip. [9.0% picked]

(D) It reports more than your weight as you start downward, then your weight as you coast through the middle floors, then less than your weight as you slow to a stop. [9.0% picked]

Answer: (B) It reports less than your weight as you start downward, then your weight as you coast through the middle floors, then more than your weight as you slow to a stop. [77.3% picked]

Why: TBA

Problem 15:

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 bounces up and down, the melon is accelerating downward whenever

(A) it is above equilibrium. [75.5% picked]

(B) it is below equilibrium. [14.6% picked]

(C) its velocity is upward. [3.9% picked]

(D) its velocity is downward. [6.0% picked]

Answer: (A) it is above equilibrium. [75.5% picked]

Why: TBA

Problem 16:

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 dog was moving forward horizontally twice as fast as the cat 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. [12.9% picked]

(B) both animals land at approximately the same time, but the dog lands considerably farther from the wall than the cat does. [82.8% picked]

(C) the dog lands first, but the cat lands considerably farther from the wall than the dog does. [1.3% picked]

(D) the dog lands first, but both animals land at approximately the same distance from the wall. [3.0% picked]

Answer: (B) both animals land at approximately the same time, but the dog lands considerably farther from the wall than the cat does. [82.8% picked]

Why: TBA

Problem 17:

A small tree behaves like a spring when it is bent away from equilibrium. When the wind exerts a 100-newton force on the tree, the tree bends 10 centimeters away from equilibrium. How far does the tree bend then the wind exerts a 200-newton force on the tree?

(A) 40 centimeters. [3.9% picked]

(B) 10 centimeters. [0.0% picked]

(C) 20 centimeters. [93.1% picked]

(D) 14.1 centimeters. [3.0% picked]

Answer: (C) 20 centimeters. [93.1% picked]

Why: TBA

Problem 18:

You must move a wagon full of library books from the first floor to the second floor. You can lift the wagon straight up a ladder or you can pull it uphill along a ramp. The ramp's surface is 4 times as long as the ladder. Compared to the work you do on the wagon lifting it up the ladder to the second floor, the work you do on the wagon pulling it uphill along the ramp to the second floor is

(A) the same. [78.1% picked]

(B) 4 times as much. [10.7% picked]

(C) 2 times as much. [2.1% picked]

(D) 1/2 as much. [9.0% picked]

Answer: (A) the same. [78.1% picked]

Why: TBA

Problem 19:

You are dropping coins into a wishing well. You drop each coin from rest and it falls to the water 9.8 meters below your hand. Assuming the acceleration due to gravity is 9.8 meters-per-second^{2}, how long does it take each coin to drop from your hand to the water? [neglect any effects due to the air.]

(A) Less than 1 second. [7.3% picked]

(B) It depends on the weight of the coin. [0.4% picked]

(C) More than 1 second. [45.9% picked]

(D) 1 second. [46.4% picked]

Answer: (C) More than 1 second. [45.9% picked]

Why: TBA

Problem 20:

You purchase a 1-kilogram bag of flour and take it to testing laboratory. Sure enough, the flour's mass is 1.0 kilograms and its weight is 9.8 newtons (about 2.2 pounds). Your astronaut friend takes the flour to the moon, where another testing laboratory examines it. They find that the flour's mass is

(A) 1.0 kilograms and its weight is much less than 9.8 newtons. [96.1% picked]

(B) much less than 1.0 kilograms and its weight is much less than 9.8 newtons. [2.6% picked]

(C) 1.0 kilograms and its weight is 9.8 newtons. [0.4% picked]

(D) much less than 1.0 kilograms and its weight is 9.8 newtons. [0.9% picked]

Answer: (A) 1.0 kilograms and its weight is much less than 9.8 newtons. [96.1% picked]

Why: TBA

Problem 21:

In baseball, the pitcher throws the baseball toward the batter and the batter swings a bat forward to hit the oncoming ball. In which of the following cases will the baseball have the fastest forward speed after being hit by the bat?

(A) When the pitcher threw the baseball toward the batter at 100 kilometers-per-hour. [1.7% picked]

(B) When the pitcher tossed the baseball slowly, so that it was barely moving when the batter hit it. [4.7% picked]

(C) The hit baseball's forward speed is the same in all of three cases. [7.3% picked]

(D) When the pitcher threw the baseball toward the batter at 160 kilometers-per-hour. [86.3% picked]

Answer: (D) When the pitcher threw the baseball toward the batter at 160 kilometers-per-hour. [86.3% picked]

Why: TBA

Problem 22:

You drop a bouncy ball straight downward onto a rigid, immovable floor. The ball bounces off the floor and rises about 70% of the way to your hand. When the ball is at its lowest height (lowest altitude) during the bounce, what are its velocity and acceleration?

(A) Its velocity is upward and its acceleration is zero. [9.0% picked]

(B) Its velocity is upward and its acceleration is upward. [8.6% picked]

(C) Its velocity is zero and its acceleration is upward. [67.8% picked]

(D) Its velocity is zero and its acceleration is zero. [14.6% picked]

Answer: (C) Its velocity is zero and its acceleration is upward. [67.8% picked]

Why: TBA

Problem 23:

You are having trouble unscrewing the lid of a jar of pickles. You attach a kitchen gadget to the lid, giving you a long handle to grip (see figure below). How should you push the handle to produce the largest torque in the direction that unscrews the lid and opens the jar?

(A) Push along Arrow M. [2.1% picked]

(B) Push along Arrow X. [71.2% picked]

(C) Push along Arrow G. [23.2% picked]

(D) Push along Arrow U. [3.4% picked]

Answer: (B) Push along Arrow X. [71.2% picked]

Why: TBA

Problem 24:

You are involved in a game of tug-o-war with a plastic rope. The two teams are pulling on opposite ends of the rope. While your team holds your end of the rope motionless, the opposing team stretches their end of the rope toward them and the rope breaks. Breaking the rope required energy and that energy was provided by

(A) neither team. It was instead provided by chemical potential energy in the rope itself. [0.0% picked]

(B) the opposing team. [89.3% picked]

(C) both teams. [9.9% picked]

(D) your team. [0.9% picked]

Answer: (B) the opposing team. [89.3% picked]

Why: TBA

Problem 25:

You raise a grocery cart upward by pushing it along a ramp at a constant uphill velocity. If the cart's altitude increases by 5 meters as it travels 20 meters along the ramp's surface (see figure below), what is the amount of the uphill force you are exerting on the cart? [neglect friction or air effects]

(A) The amount of force is 1/5 of the cart's weight. [3.9% picked]

(B) The amount of force is 1/20 of the cart's weight. [6.4% picked]

(C) The amount of force is 1/4 of the cart's weight. [89.7% picked]

(D) The amount of force is 1/80 of the cart's weight. [0.0% picked]

Answer: (C) The amount of force is 1/4 of the cart's weight. [89.7% picked]

Why: TBA

Problem 26:

You are a passenger in a car that is moving forward on level pavement. The driver brakes suddenly to avoid hitting a squirrel. As the car screeches to a stop, why does the distance between your head and the car's windshield (front window) suddenly decrease?

(A) The windshield stopped with the car but your head continued to move forward due to inertia. [94.4% picked]

(B) A frictional force from the ground pushed your head forward so that it became closer to the windshield. [0.9% picked]

(C) The force of your head's momentum pushed your head forward so that it became closer to the windshield. [3.9% picked]

(D) A support force from the seat's headrest pushed your head forward so that it became closer to the windshield. [0.9% picked]

Answer: (A) The windshield stopped with the car but your head continued to move forward due to inertia. [94.4% picked]

Why: TBA

Problem 27:

In an amusement park ride, you stand inside a hollow cylinder with your back against the wall of that cylinder. The cylinder is soon spinning rapidly at a constant angular velocity and you feel yourself pressed tightly against the wall. What is true about your motion?

(A) You are accelerating toward the center of the cylinder. [62.7% picked]

(B) Your velocity is directed toward the center of the cylinder. [16.3% picked]

(C) A strong force is pushing you outward, away from the center of the cylinder. [20.6% picked]

(D) Your speed is changing rapidly with time. [0.4% picked]

Answer: (A) You are accelerating toward the center of the cylinder. [62.7% picked]

Why: TBA

Problem 28:

A car traveling at 60 mph veers off the road and hits a tree. The car stops immediately. 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) transfers all of her momentum to whatever stops her, but that transfer is slower and involves a smaller force when she hits the airbag. [96.1% picked]

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

(C) 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]

(D) transfers more momentum to the airbag than she would have transferred to the steering wheel. [2.1% picked]

Answer: (A) transfers all of her momentum to whatever stops her, but that transfer is slower and involves a smaller force when she hits the airbag. [96.1% picked]

Why: TBA

Problem 29:

You step off a ledge and drop straight down onto a springy trampoline. You dent the trampoline deeply during your impact and then rebound high above the trampoline. When during your descent (the downward part of your motion) do you have the greatest downward velocity?

(A) When you first touch the trampoline's surface. [4.3% picked]

(B) When the trampoline is dented most deeply and you are at your lowest point of the bounce. [10.3% picked]

(C) As you pass through equilibrium on the dented trampoline. [69.5% picked]

(D) Just before you first touch the trampoline's surface. [15.9% picked]

Answer: (C) As you pass through equilibrium on the dented trampoline. [69.5% picked]

Why: TBA

Problem 30:

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 two children reach the water at the same moment and but the boy travels farther from the dock than does the girl. [14.6% picked]

(B) the girl reaches the water before the boy. [0.4% 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 boy reaches the water before the girl. [84.1% picked]

Answer: (D) the boy reaches the water before the girl. [84.1% picked]

Why: TBA