Phys214 exam#2 (30 problems in total. 5 points each, total 150 points. )

Transcription

1 Phys214 exam#2 (30 problems in total. 5 points each, total 150 points. )

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4 1. An oil tanker heading due west, straight into a strong wind, reaches a speed of 5 m/s and then shuts down its engines to drift. The forward-facing flat surfaces of the tanker catch a total of 80,000 N of force, pointing backwards, to the east, from the wind. The tanker's mass is 100,000 metric tons, which is 100 million kg. If the wind blows at this strength for 5 minutes, then how much has the wind changed the tanker's speed? A m/s B m/s C m/s D m/s 2. Suppose you are out on a frozen lake, where there is no friction. Which of the following would start you moving towards the shore? A. Shouting at someone on the shore. B. Removing a shoe and throwing it towards the shore. C. Removing a shoe and throwing it away from the shore. D. None of these would work. 3. When a single boxcar in the freight yard hooks onto a stationary string of four boxcars, as in example 7.4, there is some kinetic energy lost. Which of the following could account for the missing energy? A. Sound energy in the loud CLANG when they hook together. B. Heat energy in the clamping mechanism between cars as it opens and then captures the fifth boxcar. C. Both A and B 4. Two astronauts of equal mass are holding on to each other and moving at a speed of 40 m/s. They push off of each other and one of the astronauts moves in the same direction as the two were initially moving but at 80 m/s. The velocity of the second astronaut is A. 80 m/s backward. B. 40 m/s backward. C. zero. D. 40 m/s forward.

5 5. A bullet is fired into a wall and comes to rest. Considering the bullet as the system we can say that A. the total kinetic energy is conserved. B. the momentum is conserved. C. both total kinetic energy and momentum are conserved. D. neither total kinetic energy nor momentum is conserved. 6. In a head-on automobile collision, a person's momentum could be reduced from 2000 kg m/s to zero in just 0.1 s. If this occurs, the average force exerted to stop the person is A. 200 N. B. 2,000 N. C. 20,000 N. D. This can't be answered without knowing the person's mass. 7. Four samples of steel, lead, alcohol, and glass all have the same mass and are all initially at 20 C. After 100 calories of heat are added to each sample, the final temperatures are 38.2 C for the steel, 85.6 C for the lead, 23.4 C for the alcohol, and 30 C for the glass. Which of these four materials has the largest specific heat capacity? A. The steel. B. The lead. C. The alcohol. D. The glass. E. All have the same heat capacity, since all absorbed 100 cal of heat. 8. When a planet moves closer to the Sun in an elliptical orbit, its rotational inertia A. gets smaller because its distance from the Sun is smaller. B. gets smaller because the lever arm is shorter. C. gets larger because the angular momentum must be conserved. D. gets larger because the planet has more angular acceleration. E. does not change because the mass of the planet is constant.

6 9. A merry-go-round, having a radius of 2.5 m, is set in motion starting from rest, by students applying a force of 500 N tangential to the rim of the wheel. After five seconds of torque, the students let go and measure the rotational speed: 1.25 rad/s. The rotational inertia of the merry-go-round is A kg m 2. B kg m 2. C kg m 2. D kg m 2. E kg m An isolated object is initially spinning at a constant speed. Then, although no external forces act upon it, its rotational speed increases. This must be due to A. an increase in the moment of inertia. B. an increase in the mass. C. an increase in the angular momentum. D. a decrease in the moment of inertia. E. impossible, angular momentum conservation is violated. 11. A hydraulic jack is used to lift a load of weight 5000 N. The load-bearing cylinder has a radius of 5 cm, and the other piston has a radius of 1 cm. What force must be exerted on the smaller piston to support the load? A. 100 N B. 200 N C N D N E. 25,000 N 12. A gas originally occupies a volume of 0.5 m 3 at a pressure of 100 kpa. It is slowly allowed to expand until the volume is 2.5 m 3. Assuming the temperature is kept constant, the final pressure will be A. 10 kpa. B. 20 kpa. C. 50 kpa. D. 100 kpa. E. 500 kpa.

7 13. The density of ice is about 900 kg/m 3, and the density of water is about 1000 kg/m 3. A cubic block of ice one meter on a side floats in water. Assuming that the lowest square face of the cube is horizontal, the height of the block above the water line is A. 0.1 m. B. 0.2 m. C. 0.5 m. D. 0.8 m. E. 0.9 m. 14. On a cold winter's morning you awake and step out of bed. One foot is on the tile floor and the other is on a rug on the floor. Which statement is true? A. The tile feels colder because it is smoother than the rug. B. The tile feels colder because it conducts heat more rapidly away from your foot. C. The tile feels colder because it is connected to the cold ground outside. D. The tile feels colder because it cannot absorb water like the rug can. 15. While studying for this quiz you realize that you still have 100 g of lukewarm coffee at 40 C left in a paper cup. When you pour 50 g of boiling water into the cup, the temperature of the resulting coffee-like mixture will be now A. 50 C. B. 60 C. C. 67 C. D. 70 C. E. 80 C. 16. A hot plate is used to transfer 400 cal of heat to a beaker containing ice and water; 500 J of work are also done on the contents of the beaker by stirring. How much ice melts in this process? (latent heat: 80 cal/g. 1 cal = 4.19J). A g B g C. 6.5 g D g

8 17. If you stand near a large fire, your face will be cooler if you place your hand between your face and the fire. Your hand is preventing heat flow by A. conduction. B. convection. C. radiation. D. latent heat. 18. The space suits worn by astronauts prevent heat flows between the astronauts and their environment. In space, the kinds of heat flow that can occur are A. conduction and convection. B. conduction and radiation C. convection and radiation. D. All types of heat flow can occur.

9 19. A bullet of mass kg and speed of 100 m/s is brought to rest in a wooden block after penetrating a distance of 0.10 m. This process takes 0.02 second. The magnitude of the impulse delivered to the bullet by the block during this time is A Ns. B. 1.0 Ns. C. 2.0 Ns. D..98 N. E. zero. 20. A Carnot engine operating between reservoirs at 400 K and 300 K would have an efficiency of approximately A B C D E A heat engine having an efficiency of 0.06 takes in 2000 J of energy from the hot reservoir in one cycle. In the same time, how much work will it perform? A. 0 J B. 400 J C J D. 120 J E. 33,333 J 22. Two equal charges repel one another with a force of N when they are 10 cm apart. If they are moved until the separation is 2.5 cm, the repulsive force will be A N. B N. C N. D N. E N.

10 23. Imagine four equal positive charges, q, placed on points of a circle of radius r at the 3, 6, 9, and 12 o'clock positions. The magnitude of the electric field at the center of the circle is given by A. 4 kq/r. B. 4 kq/r 2. C. kq/r. D. kq/r 2. E. zero. 24. A uniform electric field has a magnitude of 10 N/C and is directed upward. A charge brought into the field experiences a force of 5.0 N downward. The charge must be A. +50 C. B. -50 C. C C. D C. E C. 25. A point charge creates the electric field shown. At which of the points would the potential energy of a positive test charge have the largest value? A. A B. B C. C D. D

11 26. The most common form of lightning strike from a cloud to the ground involves negative charge moving from the cloud to the ground. Just before a lightning strike, A. the potential energy of the electrons in the cloud must be small. B. the electric field must point toward the sky. C. the potential must be larger near the cloud. D. there are no charges on the ground. 27. An ice cube of mass 100 g and at 0 C is dropped into a Styrofoam cup containing 200 g of water at 25 C. The heat of fusion of ice is 80 cal/g and the specific heat capacity of water is 1.0 cal/g C. Assuming the cup does not exchange any heat with the outside, what will happen? A. All of the water will freeze. B. Some of the water will freeze. C. Nothing will happen. D. Some of the ice will melt. E. All of the ice will melt. 28. A conductor has a net charge of -24 C on it. It also has a hollow region in the interior. If you place a charge of +12 µc in this hollow region, what will be the total charge on the exterior surface of the object? A. -24 µc B. 12 µc C. 36 µc D. -12 µc E. 5 µc

12 29. Comparing the electrostatic force and the gravitational force we can say that A. both have the same dependence on distance, both involve attraction and repulsion, but the gravitational force is stronger. B. both have the same dependence on distance, both involve attraction and repulsion, but the electrostatic force is stronger. C. both have the same dependence on distance, the electrostatic force can be either attractive or repulsive while the gravitational force is only repulsive, and the electrostatic force is weaker. D. both have the same dependence on distance, the electrostatic force can be either attractive or repulsive while the gravitational force is only attractive, and the electrostatic force is stronger. E. the electrostatic force falls off more rapidly with distance, the electrostatic force can be either attractive or repulsive while gravitation is only attractive, and the electrostatic force is stronger. 30. A child is drinking a liquid with a straw. Which of the following statements is true? A. The fluid is being pulled up into the straw. B. Longer straws work better because the pressure is larger deeper down. C. The pressure inside the straw depends on the viscosity of the liquid. D. The pressure in the straw is reduced and the liquid is pushed upward by atmospheric pressure on the surface of the liquid.