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Physics 109 : How Things Work : Spring 2006 Mike Noel - [email protected] Bob Ekey - [email protected]

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Physics 109 : How Things Work : Spring 2006 Mike Noel - Mnoel@Brynmawri.Edu Bob Ekey - Rekey@Brynmawr.Edu Physics 109 : How Things Work : Spring 2006 Mike Noel - [email protected] Bob Ekey - [email protected] Practice Midterm Examination PART I: MULTIPLE CHOICE QUESTIONS Problem 1: Even when you are driving at a constant 60 miles-per-hour along a straight, level road, your car's engine must be running. As the engine turns the car's wheels, friction between the ground and the tires exerts a forward force on the car. The car needs this forward force from the ground because (A) air drag (air resistance) exerts a backward force on the car. (B) an object that is moving requires a net force to keep it moving. In the absence of any net force, objects are motionless. (C) an object's velocity points in the direction of the net force on that object. (D) the car has a velocity and is thus accelerating. In order to accelerate, the car must be experiencing a net force. Answer: (A) air drag (air resistance) exerts a backward force on the car. Why: When your car is traveling at constant velocity, the net force on it must be zero. Since it needs a forward force from the ground, something must be exerting a backward force on it. On a level road, that something can only be air resistance. Problem 2: Your pet hamster has an exercise wheel in his cage. He can climb into this wire wheel and then run in it as though he were on a treadmill. When he is running in the wheel, he remains stationary at the bottom of the wheel while it spins around him. In this case his angular velocity is (A) equal in magnitude to the wheel’s angular velocity, but in the opposite direction. (B) zero. (C) increasing as he does work on the wheel. (D) equal in both size and direction to the angular velocity of the wheel. Answer: (B) zero. Why: Since the hamster isn’t moving at all, his orientation relative to the wheel’s pivot isn’t changing. With a constant angular position, the hamster has no angular velocity. Problem 3: Burning fuel in an engine turns the ordered chemical potential energy in gasoline into disordered thermal energy in the hot burned gases. The engine's ability to extract some work from this thermal energy reflects the fact that (A) there is remaining order in an unequal distribution of temperatures: a hot region and a cold region. (B) about 25% of thermal energy is actually ordered and can be extracted as work. (C) about 10% of thermal energy is actually ordered and can be extracted as work. (D) the second law of thermodynamics is only a statistical law and doesn't always hold true. Answer: (A) there is remaining order in an unequal distribution of temperatures: a hot region and a cold region. Why: Even though the car engine is working with only thermal energy, the fact that this thermal energy is unequally distributed allows it to extract some disordered energy as ordered energy while destroying the remaining order in the situation. Problem 4: The temperature of a glass of ice water (a mixture of ice and water) (A) is proportional to the ratio of water to ice. (B) depends on the total volume of ice and water in the glass. (C) is 0 °C. (D) is proportional to the ratio of ice to water. Answer: (C) is 0 °C. Why: Ice and water can only coexist at one temperature: the freezing/melting temperature of 0 °C. Problem 5: 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. (C) denting the pillow downward 2 inches. (D) just touching the pillow on its way upward. Answer: (C) denting the pillow downward 2 inches. Why: Above the equilibrium point, the net force on your head is downward; below the equilibrium point, the net force on your head is upward. As your head falls from above the pillow, it continues to accelerate downward--picking up speed and kinetic energy--until the net force on it stops being downward. That change occurs at the equilibrium point. After the equilibrium point, your head decelerates, losing speed and kinetic energy. Problem 6: 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) you are accelerating away from the center of the merry-go-round and experience a fictitious force in the direction of your acceleration. (C) you are accelerating toward the center of the merry-go-round and experience a fictitious force in the direction opposite your acceleration. (D) your velocity is away from the center of the merry-go-round and you experience a fictitious force in the direction of your velocity. Answer: (C) you are accelerating toward the center of the merry-go-round and experience a fictitious force in the direction opposite your acceleration. Why: Left to yourself, you would go in a straight line at a steady pace. But the merry-go-round pulls inward on you to bend your path into a circle. As it does this, you accelerate inward and feel your body’s inertia trying to make you continue straight. You feel an outward pull, a fictitious force in the direction opposite your inward acceleration. Problem 7: You have an indoor swimming pool in one room of your 87-room mansion. The ventilation is turned off in that room and the air and water are at the same temperature. You observe that the amount of water in the pool remains precisely constant—water is neither evaporating nor condensing. If you were to examine the water's surface microscopically to see if any water molecules were landing on the water's surface or leaving that surface, you would find that (A) both landing and leaving are occurring, but that landing takes place more often than leaving. (B) both landing and leaving are occurring, but at exactly equal rates. (C) both landing and leaving are occurring, but that leaving takes place more often than landing. (D) both landing and leaving are absent—no molecules are landing or leaving the water. Answer: (B) both landing and leaving are occurring, but at exactly equal rates. Why: Although there is no net movement of water molecules between the liquid and the gas, that doesn't mean that the surface of the water is totally static. Instead, water molecules are landing and leaving all the time, but at equal rates. Problem 8: You wander into the kitchen on a dark, moonless night and open the refrigerator door. Sadly, the light bulb in the refrigerator has burned out and the kitchen remains pitch black. Just as you are about to reach for the carton of chocolate milk, your pet squirrel leaps onto your back and you twirl around several times before the squirrel heads off for some other part of the house. You have no idea which way you are facing. Fortunately, the refrigerator door is still open and you can feel the cold on your face as you turn in its direction. The physics explanation for this feeling of cold that you get when you face the refrigerator is that (A) heat is flowing from the rest of the kitchen into the refrigerator and the pressure of this heat flow causes coldness to accumulate on the side of your body that's facing the refrigerator. (B) the cold contents of the refrigerator do a much better job of absorbing your thermal radiation than do the warmer contents of the rest of the kitchen. (C) you are radiating far more heat toward the cold contents of the refrigerator than they are radiating toward you. (D) the cold contents of the refrigerator radiate coldness toward your skin and lower your skin's temperature. Answer: (C) you are radiating far more heat toward the cold contents of the refrigerator than they are radiating toward you. Why: Radiation only transfers heat (not cold) and the cooling effect described here is the result of an unbalanced exchange of radiation. You radiate nicely at the refrigerator, but it doesn't return the favor very well. With so little radiant heat coming toward you, you are experience a net loss of heat through radiation and feel cold. Problem 9: 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. (C) place its contact point with the ground directly below her overall center of gravity. (D) minimize the torque she exerts on it. Answer: (C) place its contact point with the ground directly below her overall center of gravity. Why: The unicycle has no static stability but can be made dynamically stable with constant effort. The cyclist tries to return the unicycle to its unstable equilibrium point by placing the support point directly under the center of gravity/mass.
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