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Physics 101 Exam 2
7 November 2003 Prof L. Weinstein

There are 16 problems. Please give a short explanation for all multiple choice questions. Show your work for all numerical answers.

G = $6.67\cdot10^{-11}$ N$\cdot$m$^2$/kg$^2$

Earth's mass $M_e = 6\cdot 10^{24}$ kg

Earth's radius $R_e = 6.4\cdot 10^6$ m

Moon's mass $M_m = 7\cdot 10^{22}$ kg

Moon's radius $R_m = 1.7\cdot 10^6$ m

Earth-Moon distance $d_{E-m} = 3.8\cdot 10^8$ m

Earth-Sun distance $d_{E-S} = 1.5\cdot10^{11}$ m

closest Earth-Jupiter distance $d_{E-J} = 6.3\cdot10^{11}$ m

Jupiter's mass $M_J = 1.9\cdot10^{27}$ kg

1. You throw a 5 kg block vertically upward with an initial kinetic energy of 30 J. What is its potential energy at the peak of its trajectory (in Joules)?
   1. 0
   2. 10
   3. 30
   4. 50
   5. need more information
   
   
   
   
   
   
   

2. In order to accelerate your 1000-kg Ford Escort from 0 to 30 m/s (about 60 mph), you need to supply the energy contained in about 1 ounce (about 25 g) of gasoline. If instead you want to accelerate a 2000-kg Explorer from 0 to 30 m/s, how much gasoline will you need?
   1. 1/4 as much
   2. 1/2 as much
   3. the same
   4. twice as much
   5. four times as much
   6. need more information
   
   
   
   
   
   
   

3. A car is moving at 30 mph, loses control, and hits the highway crash barrels (the barrels of sand that are placed near dangerous objects on the sides of highways). It stops in a distance of 2 m. If the same car was moving at a speed of 60 mph, how much distance would it need to stop? Assume that the crash barrels exert a constant force on the car.
   1. 1/2 m
   2. 1 m
   3. 2 m
   4. 4 m
   5. 8 m
   6. need more information
   
   
   
   
   
   
   

4. 75 people are all walking around. They are each walking with constant velocity at a speed that is between 1 and 5 mph. Can the group have zero total momentum? Can the group have zero total kinetic energy? Explain.

   
   
   
   
   
   
   
   
   

5. Old fashioned record players could play records that rotated at different speeds. An LP (Long Playing record) rotated at approximately once every two seconds. It had a radius of 15 cm. If you doubled the rotational speed, then the linear speed of a point on the edge of the LP would
   1. quarter
   2. halve
   3. stay the same
   4. double
   5. quadruple
   6. need more information
   
   
   
   
   
   
   

6. NASA uses centrifuges to simulate the effects of high gravity on astronauts. The centrifuge, like some amusement park rides, has a passenger cabin at the end of a long (10 m) are. The cabin then swings around in a 10-m radius circle at high speed.

   An astronaut is in the centrifuge as it rotates around. She has a linear speed $v$ and feels a centripetal force from the centrifuge. If her linear speed doubles, then the force on her from the centrifuge will

   1. quarter
   2. halve
   3. stay the same
   4. double
   5. quadruple
   6. need more information
   
   
   
   
   
   
   

7. When an object is raised off the ground to a certain height, it gains a certain amount of potential energy. If instead the same object is raised to four times the height, its potential energy will
   1. quarter
   2. halve
   3. stay the same
   4. double
   5. quadruple
   6. need more information
   
   
   
   
   
   
   

8. A car travels in a circle with constant speed. The net force on the car is
   1. directed forward, in the direction of travel
   2. directed towards the center of the circle
   3. zero because the car is not accelerating
   4. none of these
   
   
   
   
   
   
   

9. The reason that the moon does not crash into the Earth is
   1. it has a large tangential velocity
   2. the net force on it is zero
   3. it is beyond the main pull of Earth's gravity
   4. it is being pulled by the sun and the planets as well as by the Earth
   5. all of the above
   6. none of the above
   
   
   
   
   
   
   

10. Calculate the gravitational force on a newborn baby (mass = 4 kg) from the planet Jupiter (see numbers at top of exam).

    
    
    
    
    
    
    
    
    
    
    

11. If you moved to another planet with the same mass as the Earth but twice the radius, your new weight would be
    1. four times smaller
    2. two times smaller
    3. the same
    4. two times larger
    5. four times larger
    6. need more information
    
    
    
    
    
    
    

12. You are in a rotating, cylindrical, space habitat far from any solar system. Assume that there are no external forces on it. If you climb 'up' a ladder from the edge of the habitat to the center, the habitat's rotational speed will

    = 1.5in

    1. increase
    2. remain the same
    3. decrease
    4. need more information

13. When you walk, you keep your legs straight. When you run, you bend your legs at the knee. In both cases, your leg rotates around your hip (obviously, not ALL the way around). Which has a larger rotational inertia when rotated around the hip, a straight leg, or a bent leg?
    1. straight leg
    2. bent leg
    3. both the same
    4. need more information
    
    
    
    
    
    
    

14. There are two types of bicycle brakes, brakes that apply their force to the outside (rim) of the wheel and brakes that apply their force to the inside (near the axle) of the wheel. If both types of brakes can apply the same force to the wheel, which type applies more torque (and is thus more effective at stopping the bicycle)?
    1. rim brakes
    2. hub brakes
    3. both the same
    4. need more information
    
    
    
    
    
    
    

15. You use a slingshot to throw a 7 kg rock from the top of a building horizontally at a speed of 40 m/s. The rock hits the ground 2 second later. How tall is the building?

    
    
    
    
    
    
    

16. Your car gets 25 miles per gallon (mpg) when driving at 65 mph on the highway with the windows closed. If you continue driving at the same speed and turn on the air conditioner in the car, you car will get
    1. more than 25 mpg
    2. 25 mpg
    3. less than 25 mpg
    4. need more information
    
    
    
    
    
    
    

17. Two billion years ago, the moon was much closer to the Earth than it is now. At that time, the difference in water level between high tide and low tide was
    1. greater than today
    2. the same as today
    3. smaller than today
    4. need more information