Physics 105

Physics 105 - How Things Work - Fall, 2000

Midterm Examination

Given Friday, October 13, from 1:00 PM to 1:50 PM

PART I: MULTIPLE CHOICE QUESTIONS

Please mark the correct answer for each question on the bubble sheet. Fill in the dot completely with #2 pencil. Part I is worth 67% of the grade on the midterm examination.

Problem 1:

You're taking a nap on the couch. When you lower your head slowly into the elastic couch pillow and let it reach equilibrium, your head dents the pillow downward 2 inches. You lift your head to see who just walked in, and then let it fall hard against the pillow. It dents the pillow downward 4 inches before bouncing back upward. The point at which your head reaches maximum speed is when it is

(A) denting the pillow downward 4 inches.

(B) just touching the pillow on its way downward.

(D) just touching the pillow on its way upward.

Problem 2:

You are enjoying the view from 50 meters in the air as you parasail in the Bahamas during Spring Break. You are presently traveling at a constant velocity, supported by a parachute as a speedboat pulls you forward at the end of a long cable. The net force you are experience

(A) points in the forward direction, the direction in which you are heading.

(B) points toward the boat, directly along the cable that is pulling you.

(D) is zero.

Problem 3:

If you are in a car when it runs into a tree head-on and stops almost instantly, you hope that it has an airbag to protect your head. The advantage of hitting the soft airbag instead of the hard steering wheel or dashboard is that while you will still transfer all your momentum to the airbag, you will do it with

(A) less velocity and the same acceleration.

(B) less energy over a longer distance.

(D) less velocity and more acceleration.

Problem 4:

You are standing on a bathroom spring scale and decide to jump upward suddenly. As you jump, the scale reads more than your weight because

(A) you are pushing down on the scale with a force greater than your weight.

(B) your total energy increases during a jump and the scale cannot distinguish between the energy associated with your weight (your gravitational potential energy) and the energy associated with your motion (your kinetic energy).

(C) your motion fools the scale's mechanical components so that it mistakenly reports a value that is larger than the force you are exerting on it.

(D) your weight rises during an upward jump and the scale correctly reports this temporary rise in weight.

Problem 5:

When you pull a tablecloth out from under a set of dishes, it's important to pull the cloth as fast as possible because

(A) the force of sliding friction that the cloth exerts on the dishes is proportional to the time during which the cloth is moving.

(B) the work done on the dishes by the cloth is proportional to the time during which the cloth pulls on them.

(D) the momentum transferred to the dishes is proportional to the time during which the cloth pulls on them.

Problem 6:

You accidentally bump into a bookcase. It tilts briefly but then returns to upright, and you breathe a sigh of relief. When you first bumped the bookcase, its center of gravity

(A) moved upward and its gravitational potential energy increased.

(B) moved upward and its gravitational potential energy decreased.

(D) moved downward and its gravitational potential energy decreased.

Problem 7:

When an acrobat tries to balance on top of a unicycle, she pedals the single wheel so as to

(A) minimize her overall gravitational potential energy.

(B) accelerate at a constant rate and minimize her velocity.

(D) minimize the torque she exerts on it.

Problem 8:

Hockey star Jaromir Jagr hits a fast wrist shot to the upper right corner of the goal, 5 meters (17 feet) away, and scores! After the hockey puck leaves Jagr's stick, the puck is experiencing

(A) no forward force.

(B) a constant forward force that diminishes only when the puck hits the net after passing through the goal.

(C) a forward force that diminishes uniformly with distance but is still strong enough to knock the net backward after the puck passes through the goal.

(D) a forward force that diminishes uniformly with distance and reaches zero just as it hits the net behind the goal.

Problem 9:

Your apartment window opens 20 meters (65 feet) above a lemonade stand. Your friend lowers a long plastic tube out the window until its end enters the tank of delicious lemonade far below. She then begins to suck on the other end of the tube in hopes of getting a free drink. To her dismay, she never tastes a drop because

(A) atmospheric pressure cannot support a column of lemonade 20 meters tall.

(B) lemonade has too high a viscosity to pass through a tube that long.

(D) the tube's acceleration is downward, so it prevents the lemonade's velocity from being upward.

Problem 10:

You're seated at a table in a Paris Cafe, contemplating life and watching the bubbles rise upward in your glass of Perrier (carbonated water). You can think of many physical reasons why those carbon dioxide bubbles should rise upward through the water. Which of the follow observations IS NOT TRUE and therefore DOES NOT HELP EXPLAIN why the bubbles rise in your Perrier?

(A) A bubble displaces more than its weight of water.

(B) The pressure below a bubble is greater than the pressure above it.

(D) A bubble's average density is less than that of water.

Problem 11:

One of the strength training machines in the exercise facility has an interesting behavior: as you lift its handles upward, you must push up with a force of 450 newtons (100 pounds). But as you lower its handles back downward, you must push up with a force of 600 newtons (133 pounds). During your exercise routine, you move these handles slowly up and down a dozen times, and leave them exactly where you found them. As the result of this routine, your total energy

(A) stays the same but you convert food energy into thermal energy.

(B) increases.

(D) stays the same but you convert thermal energy into food energy.

Problem 12:

Tiger Woods has just driven the golf ball toward the green in a long, nearly horizontal shot through the air. Despite the air resistance it encounters, the ball is on track to hit only feet from the hole. When the ball is exactly half way through its flight, its velocity is

(A) backward (away from the green) and its acceleration is backward (away from the green).

(B) backward (away from the green) and its acceleration is forward (toward the green).

(D) forward (toward the green) and its acceleration is backward (away from the green).

Problem 13:

As you ride on a merry-go-round, you feel a strong outward pull that feels just like the force of gravity. This fictitious force occurs because

(A) your velocity is toward the center of the merry-go-round and you experience a fictitious force in the direction opposite your velocity.

(B) your velocity is away from the center of the merry-go-round and you experience a fictitious force in the direction of your velocity.

(C) you are accelerating toward the center of the merry-go-round and experience a fictitious force in the direction opposite your acceleration.

(D) you are accelerating away from the center of the merry-go-round and experience a fictitious force in the direction of your acceleration.

Problem 14:

You're sitting on a park bench while your dog walks at the end of a spring-loaded leash. This leash emerges from a plastic container with a handle and can extend up to 5 meters (17 feet) if the dog pulls on it hard enough. As the leash extends outward, a spring in the container stretches. When the dog stops pulling, that spring then retracts the leash back into the container. As the dog pulls the leash outward, stretching the spring, she does work on the leash. During which meter of extension does the dog do the most work on the leash?

(A) During the third meter--when the leash is at the middle of its extension.

(B) During the fifth meter--just before the leash is fully extended.

(D) The work the dog does is the same during each of the five meters of extension.

Problem 15:

You are practicing shooting free throws at the basketball court. When you throw the ball, it travels in an arc toward the hoop. Ignoring any forces that air exerts on the ball, the net force on the ball just after it leaves you hand is

(A) down and away from you.

(B) straight down.

(D) up and away from you.

Problem 16:

You're at the county fair and you're trying to win a stuffed animal by knocking over a stack of heavy milk bottles with a projectile. You have a choice of four projectiles: a 1-kilogram beanbag, a 2-kilogram beanbag, a 1-kilogram superball, and a 2-kilogram superball. Assuming that you can throw each of these projectiles at the same final speed, which one will be most effective at knocking over the milk bottles?

(A) The 2-kilogram superball.

(B) The 1-kilogram beanbag.

(D) The 1-kilogram superball.

Problem 17:

You are out in space, so far from any star or planet that gravity is insignificant. You throw two rubber balls so that they drift forward as a pair. These balls continue to touch one another with one ball directly in front of the other. Which of the balls is pushing on the other?

(A) The ball in front is pushing backward on the ball behind and the ball behind is pushing forward on the ball in front.

(B) Neither ball is pushing on the other.

(D) Only the ball behind is pushing forward on the ball in front.

Problem 18:

You are in a store that sells bathroom scales and decide to stand on two spring scales at once. Both scales are resting on the ground and you stand stationary with one foot on each scale. You can determine your correct weight by

(A) taking the reading from either scale and doubling it.

(B) taking the reading from either scale and dividing it in half.

(D) adding together the readings of the two scales.

Problem 19:

When a log is floating on water, much of the log is above the water and is actually surrounded by air. If that surrounding air were to suddenly disappear, the log would

(A) float at the same height as before the air left.

(B) move downward slightly and float somewhat lower (deeper) in the water.

(D) move upward slightly and float somewhat higher (less deep) in the water.

Problem 20:

Your gourmet foods business has begun exporting to the moon. Some of your chocolates are sold in 1-kilogram boxes while others are sold in 1-pound boxes. Both types of boxes are correctly labeled at the manufacturing plant on the earth. At the customs station on the moon, officers check your chocolates to make sure that they are properly labeled. After a routine inspection, a customs officer reports that

(A) the 1-kilogram boxes are labeled correctly but the 1-pound boxes are not.

(B) the 1-kilogram boxes are not labeled correctly but the 1-pound boxes are.

(D) neither the 1-kilogram boxes nor the 1-pound boxes are labeled correctly.

Problem 21:

The top surface of a calm, smoothly flowing stream is always at atmospheric pressure. As water in this stream runs into a tree stump and slows almost to a stop, the water's top surface

(A) shifts upward slightly above the normal stream level.

(B) shifts downward slightly below the normal stream level.

(D) stays at the same height but begins to rotate clockwise as viewed from above.

Problem 22:

While a superball bounces extremely well on a hardwood floor, it bounces poorly on a sandy beach. In contrast, a beach ball (a thin air-filled ball) bounces about the same from either surface. The beach ball's bounce is almost independent of what surface it hits because the beach ball

(A) has almost no net force on it and therefore isn't strongly affected by its impact with a surface.

(B) is much more lively than almost any surface it hits and is thus responsible for most of the rebound energy.

(D) experiences a large buoyant force that doesn't depend on the surface it hits.

Problem 23:

You're hanging a plant in your bedroom on the moon and you're about to pound a nail into the ceiling. Gravity is weaker on the moon than on earth. You take your hammer and swing it upward so that it hits the nail above it. The head of your hammer reaches exactly its usual speed just before it hits the nail. Compared to when you hung a plant in your bedroom on earth, did it take less torque or more torque to swing the hammer's head upward to its usual speed on the moon, and was the hammer's head more or less effective at driving in the nail on the moon?

(A) On the moon, it took more torque to swing the hammer upward and the hammer's head was less effective at driving the nail.

(B) On the moon, it took less torque to swing the hammer upward and the hammer's head was less effective at driving the nail.

(C) On the moon, it took less torque to swing the hammer upward and the hammer's head was more effective at driving the nail.

(D) On the moon, it took more torque to swing the hammer upward and the hammer's head was more effective at driving the nail.

Problem 24:

When you are out fishing on a calm lake one day, you get your line snagged on a tree branch floating in the viscous water. Luckily the branch is not attached to anything, so you can reel it in to unhook your line. As you reel in the branch, it moves toward you at a constant speed. The amount of work you are doing on the branch is

(A) constant, but negative--the branch does work on you.

(B) exactly equal to the kinetic energy of the branch.

(D) constant and positive--you do work on the branch.

Problem 25:

Suppose you are standing motionless on a frictionless skateboard and are trying to propel yourself forward by throwing tennis balls at the wall behind you. You throw one of the balls and soon find yourself heading forward. You first begin to head forward

(A) when the tennis ball comes to a complete stop during its bounce off the wall.

(B) when the tennis ball just begins to touch the wall at the start of its bounce off the wall.

(D) when the tennis ball just finishes bouncing off the wall.

PART II: SHORT ANSWER QUESTIONS

Please give a brief answer in the space provided. Part II is worth 33% of the grade on the midterm examination.

Problem 1:

At the start of a kayak race, you're sitting motionless in your kayak and the kayak is motionless on the water.

(A) How much force is the water exerting on the kayak (give both the amount of that force and its direction).

(B)_ The starting gun goes off and you pull your paddle rapidly through the water, from the front of the boat to the rear. Your kayak accelerates forward. What force is causing the kayak to accelerate forward?

(C)_ At one point in the course, you notice that water is flowing into a hole in the rocks and its pressure increases as it enters that hole. You recall that water normally accelerates toward lower pressure. How can the water enter this hole if the pressure there is higher than outside the hole?

(D) You go over a small waterfall with the flowing stream. How are the water's three forms of energy changing at the surface of this water as it falls downward?

Problem 2:

Pine derby cars are small wooden cars that roll down a gently curving track that starts with a fairly steep slope and ends up traveling horizontally for several meters. The cars are released from rest and the first one that crosses the finish line at the bottom of the hill wins.

(A) Assuming no air resistance or friction, during which part of its trip does the car have its greatest speed?

(B)_ Assuming no air resistance or friction, during which part of its trip does the car have its greatest acceleration?

(C)_ In reality, friction does affect these little cars. The plastic car wheels spin on fine, stationary wire axles. There is friction between each wheel and the ground, and there is friction between each wheel and the axle that supports it. Why is more energy wasted by friction between the wheel and axle than by friction between the wheel and ground?

(D) You must put a 20-gram weight on your car to reach the required racing weight. If you want your car to travel as fast as possible, roughly where on the car should you put that weight? (This is a challenging question that I have been asked dozens of times by the parents of children with pine derby cars. If you think in terms of energy, you should be able to answer this question.)

Problem 3:

Your pet hamster runs in a wire wheel that's suspended by a horizontal axle passing through the middle of the wheel. This wheel is oriented and suspended much like the wheel of an upright bicycle.

(A) When the hamster first steps into the wheel, the wheel swings back and forth before finally settling down with the hamster at the lowest possible point. If the hamster isn't at the lowest possible point, the wheel undergoes angular acceleration in the direction that will return him to that lowest point. Use energy concepts to explain why the wheel undergoes angular acceleration in that direction.

(B)_ If the hamster tries to run forward and climb up the inside of the wheel, the wheel will begin turning faster and faster so as to return the hamster to the lowest possible point. If the hamster then suddenly stops running, the wheel will continue to spin anyway. What keeps the wheel spinning even though the hamster is no longer trying to spin it?

(C)_ If the wheel was spinning fast enough, it will carry the tired hamster all the way around in a full circle. The hamster travels backward, up, forward, and down--one complete circle. As the hamster goes over the top of this unexpected loop-the-loop, in which direction is the hamster accelerating?

(D) If the hamster closes its eyes, it will not be aware that it has traveled upside-down. As it goes over the top, it will feel as though gravity is pulling it upward toward the sky. It will find itself pressed hard against the wire wheel even though the wheel's surface is actually above it. Compare the magnitude of the hamster's acceleration as it travels over the top of the circle to the acceleration due to gravity.