Midterm Examination 2 Solutions

Problem 1:

An unopened glass bottle of soda or beer is not quite full of liquid and there is some air in the bottle's neck. If you grip the bottle's sides and strike the bottle's cap hard with a mallet or book, what will happen to the bottle?

(A) The bottle's sides will collapse inward. [46.4% picked]

(B) The liquid will knock the bottom out of the bottle. [45.9% picked]

(C) The bottle's neck will collapse downward. [0.4% picked]

(D) The bottle's cap will jump upward to the ceiling. [7.3% picked]

Answer: (B) The liquid will knock the bottom out of the bottle. [45.9% picked]

Why: TBA

Problem 2:

After boarding a bus that is stopped at a bus stop, you stand motionless on your skateboard in the middle of the bus. As the bus then 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 backward and equal in amount to the velocity of the bus. [3.0% picked]

(B) Your velocity is zero. [82.8% picked]

(C) Your velocity is forward and equal in amount to half the velocity of the bus. [12.9% picked]

(D) Your velocity is backward and equal in amount to half the velocity of the bus. [1.3% picked]

Answer: (B) Your velocity is zero. [82.8% picked]

Why: TBA

Problem 3:

You are on a 10th floor hotel balcony, overlooking the pool. You lower a 100-foot-long straw into the pitcher of lemonade at the bar and try to suck the lemonade up to your mouth. What happens?

(A) You succeed in drinking the lemonade almost immediately, but it flows into your mouth at a very slow rate. [5.6% picked]

(B) You succeed in drinking the lemonade at a normal rate, but it takes a long time for the lemonade to first reach your mouth. [33.6% picked]

(C) You succeed in drinking the lemonade, but it takes a long time for the lemonade to first reach your mouth and then flows into your mouth at a very slow rate. [2.2% picked]

(D) You are unable to suck the lemonade up to your mouth. [58.6% picked]

Answer: (D) You are unable to suck the lemonade up to your mouth. [58.6% picked]

Why: TBA

Problem 4:

You are cleaning a wall by spraying water at it from a hose. At the center of the stream of water, right where it hits the wall, the water is coming to a complete stop. If you were to measure the water pressure at that point, you would find that it is

(A) equal to atmospheric pressure. [12.9% picked]

(B) exactly zero. [20.2% picked]

(C) lower than atmospheric pressure, but more than zero. [1.7% picked]

(D) higher than atmospheric pressure. [65.2% picked]

Answer: (D) higher than atmospheric pressure. [65.2% picked]

Why: TBA

Problem 5:

To propel your boat forward on a calm lake, you and your teammates move the flat blades of oars backward through the water. The boat moves forward at constant velocity. What significant forces is water exerting on the boat and oars?

(A) Water exerts a forward buoyant force on the oars, an upward buoyant force on the boat, and a backward frictional force on the boat. [9.0% picked]

(B) Water exerts an upward support force on the boat and a forward support force on the oars. [3.0% picked]

(C) Water exerts forward gravitational forces on the oars and a downward gravitational force on the boat. [4.7% picked]

(D) Water exerts forward pressure drag forces on the oars, a backward pressure drag force on the boat, and upward buoyant forces on the boat and oars. [83.3% picked]

Answer: (D) Water exerts forward pressure drag forces on the oars, a backward pressure drag force on the boat, and upward buoyant forces on the boat and oars. [83.3% picked]

Why: TBA

Problem 6:

To mix creamer into your tea or coffee, you move a teaspoon through the beverage at about 5 inches (12.5 cm) per second. The beverage becomes turbulent and mixing is rapid. If you use the same technique to mix two salad oils together, it's less effective. Why?

(A) Salad oils are slippery and do not interact with the teaspoon. [2.6% picked]

(B) Salad oils are denser than tea or coffee and you must stir them faster to cause turbulence. [0.4% picked]

(C) Salad oils are more viscous than tea or coffee and you must stir them faster to cause turbulence. [96.1% picked]

(D) Salad oils are clear and do not mix as well as well as dark or opaque liquids. [0.9% picked]

Answer: (C) Salad oils are more viscous than tea or coffee and you must stir them faster to cause turbulence. [96.1% picked]

Why: TBA

Problem 7:

Water in a garden hose flows through a nozzle and into the air. How do the water's pressure and total energy change as the water passes through the nozzle? [neglect effects due to viscosity and assume laminar flow throughout]

(A) The water's pressure decreases but its total energy remains constant. [62.7% picked]

(B) The water's pressure increases and its total energy decreases. [3.4% picked]

(C) The water's pressure increases but its total energy remains constant. [0.9% picked]

(D) The water's pressure decreases and its total energy decreases. [33.0% picked]

Answer: (A) The water's pressure decreases but its total energy remains constant. [62.7% picked]

Why: TBA

Problem 8:

As you blow up a balloon, the balloon's total weight

(A) remains constant and the buoyant force on the balloon increases. [38.6% picked]

(B) increases and the buoyant force on the balloon increases. [42.5% picked]

(C) increases and the buoyant force on the balloon remains constant. [12.4% picked]

(D) remains constant and the buoyant force on the balloon remains constant. [6.4% picked]

Answer: (B) increases and the buoyant force on the balloon increases. [42.5% picked]

Why: TBA

Problem 9:

A hanging sculpture at the art gallery consists of a dozen metal pieces attached to one another by strings. The sculpture is motionless. When you disturb it, it moves about briefly but soon settles back in its original arrangement and is motionless again. The sculpture tends to return to that arrangement because that arrangement has

(A) the most total potential energy possible for the sculpture. [0.9% picked]

(B) the least kinetic energy possible for the sculpture. [0.0% picked]

(C) the most kinetic energy possible for the sculpture. [9.9% picked]

(D) the least total potential energy possible for the sculpture. [89.3% picked]

Answer: (D) the least total potential energy possible for the sculpture. [89.3% picked]

Why: TBA

Problem 10:

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) 1 second. [0.0% picked]

(B) It depends on the weight of the coin. [2.1% picked]

(C) Less than 1 second. [16.7% picked]

(D) More than 1 second. [81.1% picked]

Answer: (D) More than 1 second. [81.1% picked]

Why: TBA

Problem 11:

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 mph). 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 20 meters-per-second. [2.1% picked]

(B) 10-meters-per-second. [3.4% picked]

(C) More than 10 meters-per-second, but less than 20 meters-per-second. [71.2% picked]

(D) 20 meters-per-second. [23.2% picked]

Answer: (C) More than 10 meters-per-second, but less than 20 meters-per-second. [71.2% picked]

Why: TBA

Problem 12:

You seal a rigid glass bottle full of room-temperature air and put the bottle in the freezer. When you take the ice-cold bottle out of the freezer an hour later, the air in the bottle has

(A) the same weight and the same density as before, but its pressure has decreased. [87.1% picked]

(B) the same pressure, but its weight has increased and its density has decreased. [3.9% picked]

(C) the same pressure, but its weight has decreased and its density has increased. [3.4% picked]

(D) the same weight and the same pressure, but its density has increased. [5.6% picked]

Answer: (A) the same weight and the same density as before, but its pressure has decreased. [87.1% picked]

Why: TBA

Problem 13:

At launch, a rocket consists of spaceship and fuel. Once the rocket has used all of its fuel, how fast can the spaceship be moving?

(A) The spaceship's speed can be up to half the exhaust velocity of its rocket engine. [0.4% picked]

(B) There is no limit to the spaceship's speed if the spaceship-fraction of the rocket's launch mass was small enough. [96.1% picked]

(C) The spaceship's speed can be up to twice the exhaust velocity of its rocket engine. [1.3% picked]

(D) The spaceship's speed can be up to the exhaust velocity of its rocket engine. [2.1% picked]

Answer: (B) There is no limit to the spaceship's speed if the spaceship-fraction of the rocket's launch mass was small enough. [96.1% picked]

Why: TBA

Problem 14:

A spaceship is traveling in a circular orbit at an altitude of 200 kilometers above the ground. The ship's rocket engine fires briefly, sending a plume of rocket exhaust forward—in the same direction as the ship's current velocity. Immediately after that rocket use, the ship's speed has

(A) decreased and it starts moving to lower altitude. [89.7% picked]

(B) decreased and it starts moving to higher altitude. [6.4% picked]

(C) increased and it starts moving to higher altitude. [3.9% picked]

(D) increased and it starts moving to lower altitude. [0.0% picked]

Answer: (A) decreased and it starts moving to lower altitude. [89.7% picked]

Why: TBA

Problem 15:

A carpenter sometimes uses a syringe with a needle to inject glue into the joints of a wooden project. The glue is viscous (thick) and it takes several minutes to fill each joint through a narrow needle. The carpenter replaces the first needle with one that is twice the diameter. How does doubling the diameter of the needle affect the time required to fill each joint?

(A) Each joint fills 16 times as quickly as with the first needle. [94.4% picked]

(B) Each joint fills 2 times as quickly as with the first needle. [3.9% picked]

(C) Each joint fills 4 times as quickly as with the first needle. [0.9% picked]

(D) Each joint fills 8 times as quickly as with the first needle. [0.9% picked]

Answer: (A) Each joint fills 16 times as quickly as with the first needle. [94.4% picked]

Why: TBA

Problem 16:

After crossing the finish line at top speed, a runner leans backward as she slows quickly to a stop. That backward lean puts her feet ahead of her upper body. Why does she lean?

(A) The runner accelerates in the direction of her lean. She leans forward to speed up at the beginning of the race and she leans backward to slow down at the end of the race. [3.0% picked]

(B) To get rid of her forward momentum, she transfers it to her upper body by leaning backward. [0.4% picked]

(C) To get rid of her kinetic energy, she converts it into gravitational potential energy in her upper body by leaning backward. [2.6% picked]

(D) The ground is exerting a backward frictional force on her feet, producing a torque about her center of mass. To counteract that torque and avoid falling head first, she has to lean backward. [94.0% picked]

Answer: (D) The ground is exerting a backward frictional force on her feet, producing a torque about her center of mass. To counteract that torque and avoid falling head first, she has to lean backward. [94.0% picked]

Why: TBA

Problem 17:

As water travels upward through the air in a fountain, what happens to the water's pressure and speed?

(A) The water's pressure decreases and its speed decreases. [17.2% picked]

(B) The water's pressure is constant and its speed is constant. [58.8% picked]

(C) The water's pressure decreases and its speed is constant. [6.4% picked]

(D) The water's pressure is constant and its speed decreases. [17.6% picked]

Answer: (D) The water's pressure is constant and its speed decreases. [17.6% picked]

Why: TBA

Problem 18:

As a prank, someone has thrown toilet paper into your neighbors' trees. The neighbors are trying to get it out with the jet of water, using a nozzle on a garden hose. When they stand on the ground, the water jet rises to only 3/4 the height of the tallest trees. They raise the nozzle 10 meters upward by standing on the roof of their house and the water jet rises [neglect any effects due to viscosity]

(A) to 10 meters higher than before. [6.4% picked]

(B) to 20 meters higher than before. [28.3% picked]

(C) to the same height as before. [58.4% picked]

(D) to 10 meters lower than before. [6.9% picked]

Answer: (C) to the same height as before. [58.4% picked]

Why: TBA

Problem 19:

You and your friend are riding a carousel. Your horses are side-by-side and your friend is closer to the center of the carousel than you are. As the carousel turns steadily, which of the following is true?

(A) You and your friend experience equal inward accelerations, but your friend's speed is larger than your speed. [26.2% picked]

(B) You and your friend experience equal inward accelerations, but your speed is larger than your friend's speed. [8.2% picked]

(C) Your friend experiences a larger inward acceleration than you do. [2.1% picked]

(D) You experience a larger inward acceleration than your friend does. [63.5% picked]

Answer: (D) You experience a larger inward acceleration than your friend does. [63.5% picked]

Why: TBA

Problem 20:

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 largest when it is descending. [3.0% picked]

(B) Its acceleration is largest when it is at peak height. [3.9% picked]

(C) Its acceleration is constant throughout the arc. [93.1% picked]

(D) Its acceleration is largest when it is rising. [0.0% picked]

Answer: (C) Its acceleration is constant throughout the arc. [93.1% picked]

Why: TBA

Problem 21:

Walking on a tightrope is difficult, even though the walker is usually at equilibrium. Why?

(A) Gravity exerts a torque on the walker about the walker's center of mass and tends to cause the walker to begin rotating. [1.7% picked]

(B) At equilibrium, the walker has zero net force and thus cannot move. [7.3% picked]

(C) The equilibrium is unstable, so any disturbance from that equilibrium tends to cause acceleration away from equilibrium. [86.3% picked]

(D) At equilibrium, the rope pushes upward on the walker's feet and produces a torque about the walker's center of mass. [4.7% picked]

Answer: (C) The equilibrium is unstable, so any disturbance from that equilibrium tends to cause acceleration away from equilibrium. [86.3% picked]

Why: TBA

Problem 22:

A speedboat is traveling across a lake. You notice that the water's smooth surface rises upward slightly at the boat's front and drops slightly around the boat's curved sides. Why?

(A) The buoyant force is stronger at the front of the boat and lifts the water there upward. The buoyant force then decreases around the sides of the boat and the water there falls downward. [15.9% picked]

(B) Archimedes' principle requires that the water level at the front of an object be higher than the water level at the sides of the object. [10.3% picked]

(C) The force of the water's momentum lifts the water upward as it encounters the boat's front. Having lost most of its momentum, the water falls downward to less than its original height at the boat's sides. [4.3% picked]

(D) To bend water away from the boat's front, the pressure there increases and can support more water above it. To bend water toward the boat's sides, the pressure there decreases and can support less water above it. [69.5% picked]

Answer: (D) To bend water away from the boat's front, the pressure there increases and can support more water above it. To bend water toward the boat's sides, the pressure there decreases and can support less water above it. [69.5% picked]

Why: TBA

Problem 23:

After fighting a fire with a huge flow of water, the firefighters suddenly close the fire hydrant. The water in the water main (underground pipe) stops suddenly. While the water is stopping, the pressure in the water main near the fire hydrant

(A) increases by about 10%. [1.7% picked]

(B) increases dramatically (to more than double). [74.2% picked]

(C) decreases dramatically (to less than half). [22.7% picked]

(D) decreases by about 10%. [1.3% picked]

Answer: (B) increases dramatically (to more than double). [74.2% picked]

Why: TBA

Problem 24:

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) there is too little time for the paper's frictional forces to transfer significant rightward momentum to the coins. [84.1% picked]

(B) the force of each coin's momentum pushes it oppose the frictional force of the paper and those two forces cancel. [14.6% picked]

(C) the static frictional forces that the paper exerts on the coins are too weak to cause them to accelerate. [0.4% picked]

(D) the sliding frictional forces that the paper exerts on the coins are too weak to cause the coins to accelerate. [0.9% picked]

Answer: (A) there is too little time for the paper's frictional forces to transfer significant rightward momentum to the coins. [84.1% picked]

Why: TBA

Problem 25:

Two plastic balloons are loosely filled with air and have equal volumes. The cool balloon is filled with cool air and the hot balloon is filled with hot air. Compare density, weight, and number of air particles in the two balloons.

(A) The cool balloon's density and number of air particles are greater than those of the hot balloon, but the two balloons have equal weights. [3.0% picked]

(B) The cool balloon's density and weight are greater than those of the hot balloon, but the two balloons contain equal numbers of air particles. [11.2% picked]

(C) The cool balloon's density, weight, and number of air particles are all greater than those of the hot balloon. [65.1% picked]

(D) The cool balloon's weight and number of air particles are greater than those of the hot balloon, but the two balloons have equal densities. [20.7% picked]

Answer: (C) The cool balloon's density, weight, and number of air particles are all greater than those of the hot balloon. [65.1% picked]

Why: TBA

Problem 26:

If you're trying to increase the pressure in the water distribution system by modifying the local water tower, you should make the water tower [neglect effects due to viscosity]

(A) shorter. [16.3% picked]

(B) narrower. [20.6% picked]

(C) taller. [62.7% picked]

(D) wider. [0.4% picked]

Answer: (C) taller. [62.7% picked]

Why: TBA

Problem 27:

A toy boat is floating motionless on a calm pond in your backyard. You pour carbon dioxide gas, which is denser than air, onto the surface of the pond. Because the air around the boat has been replaced by carbon dioxide, the boat

(A) moves downward due to the mass of the carbon dioxide and more of the boat is in the water. [9.0% picked]

(B) continues to float at the same height, with the same amount of the boat in the water. [8.6% picked]

(C) moves upward and less of the boat is in water. [67.8% picked]

(D) moves downward due to the weight of the carbon dioxide and more of the boat is in water. [14.6% picked]

Answer: (C) moves upward and less of the boat is in water. [67.8% picked]

Why: TBA

Problem 28:

During a bicycle race on level pavement, you and another bicyclist are traveling side-by-side at the same constant velocity. You shift your position to travel directly behind the other bicyclist and find that you don't have to pedal as hard maintain the same constant velocity. Why?

(A) The air around you has been dragged forward by the other bicyclist and you experience milder pressure drag. [78.1% picked]

(B) Since your bicycle's wheels are rolling on the same pavement as the bicycle in front of you, your bicycle's wheels experience more forward frictional force. [10.7% picked]

(C) Having the other bicyclist directly in front of you reduces the total potential energy of your bicycle so that you accelerate forward. [9.0% picked]

(D) The other bicyclist exerts an attractive force on your bicycle and reduces the forward frictional force your bicycle must obtain from the pavement. [2.1% picked]

Answer: (A) The air around you has been dragged forward by the other bicyclist and you experience milder pressure drag. [78.1% picked]

Why: TBA

Problem 29:

To wash your clothes, you put them in the cylindrical drum of a washing machine. After washing the clothes, the machine removes most of the wash water from the clothes by spinning the drum rapidly. The water flows out of the drum through perforations and is discarded. What force pushes the water out of the spinning drum?

(A) No force pushes the water outward. Because of inertia, the water tends to travel straight and thus drifts out of the drum. [77.3% picked]

(B) An outward gravitational force pulls the water out of the clothes and through the drum's perforations. [9.0% picked]

(C) A support force from the drum pushes outward on the water and propels the water through the perforations. [9.0% picked]

(D) The outward force is exerted by the washer's motor, which also pushes outward on the clothes to press them against the drum. [4.7% picked]

Answer: (A) No force pushes the water outward. Because of inertia, the water tends to travel straight and thus drifts out of the drum. [77.3% picked]

Why: TBA

Problem 30:

During a spacewalk, an astronaut becomes separated from her spaceship. Her momentum is directed away from the ship and that's not good. Fortunately, she is holding a wrench and she throws it away from the ship as hard as she can. She coasts toward the ship and is soon safe. Describe the total momentum of the astronaut and wrench during her return to safety.

(A) The total momentum is zero, though her momentum is away from the ship and the wrench's momentum is toward the ship. [6.0% picked]

(B) The total momentum is still directed away from the ship. [75.5% picked]

(C) The total momentum is zero, though her momentum is toward the ship and the wrench's momentum is away from the ship. [14.6% picked]

(D) The total momentum is direct toward the ship. [3.9% picked]

Answer: (B) The total momentum is still directed away from the ship. [75.5% picked]

Why: TBA