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Physics 101 Exam 2
8 November 2002 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

1. Which has more potential energy, a 2 kg rock at a height of 3 m or a 1 kg rock at a height of 6 m?
   1. the 2 kg rock
   2. the 1 kg rock
   3. both the same
   4. need more info
   
   
   
   
   
   
   

2. Which rock will have a greater speed when it hits the ground, a 2 kg rock dropped from a height of 3 m or a 1 kg rock dropped from a height of 6 m?
   1. the 2 kg rock
   2. the 1 kg rock
   3. both the same
   4. need more info
   
   
   
   
   
   
   

3. You throw two balls in the air at the same time. They follow the trajectories shown. Ignore air resistance. Which ball hits the ground first?

   = 1.5in

   1. A
   2. B
   3. both the same
   4. need more information

4. My 40 kg son likes to sit on his skate board and be pushed. If I pushed him for a distance of 5 m while applying a horizontal force of 100 N, how much work did I do?

   
   
   
   
   
   
   

5. In the previous problem, if I push him for 5 m, he will attain a certain speed $v$. If instead I push him for 10 m, his new speed will be (ignoring friction)
   1. less than $v$
   2. $v$ (ie: the same)
   3. between $v$ and $2v$
   4. $2v$ (ie: doubled)
   5. more than $2v$
   
   
   
   
   

6. If the sun doubled its radius from 1 million km to 2 million km without changing its mass, then the gravitational force between the sun and the Earth would
   1. increase
   2. decrease
   3. stay the same
   4. need more information
   
   
   
   
   
   
   

7. Norfolk is at a latitude of about 40 degrees N. Our linear speed due to the rotation of the Earth about its axis is about 350 m/s. If you move to the equator your linear speed due to the rotation of the Earth about its axis will
   1. increase
   2. decrease
   3. remain the same
   4. need more information
   
   
   
   
   

8. A driver of mass 70 kg is driving her red 2000 kg car at 12 m/s. She enters the highway and speeds up to 24 m/s. When she goes from 12 m/s to 24 m/s her kinetic energy
   1. halves
   2. stays the same
   3. doubles
   4. quadruples
   5. other
   
   
   
   
   

9. A common playground item (in the days before lawsuits) used to be an unpowered merry-go-round. This is a circular platform that is free to rotate. Some children get on the merry-go-round while others push it to make it rotate. Suppose that there are 3 children on it standing near the edge of the platform and it is rotating at 7 revolutions per minute after everyone has stopped pushing. When the children move to the middle of the platform, the rotational speed

   = 1.2in

   1. increases
   2. decreases
   3. remains the same
   4. need more information

10. You throw a 2.3 kg rock horizontally from the top of Mount Trashmore and it hits the ground 75 m away from you and 20 m below you. How much time (in seconds) did it take to fall?

    
    
    
    
    
    
    

11. If I drive my 1000 kg car around a curve at 30 mph, my car needs a certain amount of centripetal force to stay on the curve and not leave the road. If I drive around the same curve at 60 mph, then to stay on the road my car needs
    1. less force
    2. the same force
    3. twice as much force
    4. four times as much force
    5. need more information
    
    
    
    
    

12. The tidal force difference of the moon on the Earth is due to the difference in gravitational attraction of the moon on the near side of the Earth and on the far side of the Earth. If the Earth's radius doubled without changing its mass, the tidal force difference on the Earth from the moon would be
    1. smaller
    2. the same
    3. larger
    4. need more information
    
    
    
    
    

13. The minimum orbital speed at a distance of 6600 km from the center of the Earth (ie: near the Earth's surface) is 8 km/s. What is the minimum orbital speed at a distance of 10,000 km from the center of the Earth?
    1. less than 8 km/s
    2. 8 km/s
    3. more than 8 km/s
    4. need more information
    
    
    
    
    
    
    

14. Normally, you push on the side of a door farthest from the hinges to make it open. If instead, you push on the middle of the door, to apply the same torque to the door (ie: to open it as quickly), you need to apply
    1. less force
    2. the same force
    3. more force
    4. need more information
    
    
    
    
    
    
    

15. Astronauts in the space shuttle in orbit 250 km above the surface of the Earth appear weightless because
    1. the net force on them is zero
    2. there is no gravitational pull from the Earth
    3. they are above the atmosphere
    4. the moon's gravity pulls on them too
    5. other
    
    
    
    
    
    
    

16. A box, with its center of mass off center, as indicated by the black dot, is placed on an inclined plane. In which of the four orientations shown, if any, does the box tip over? (You may select more than one.)

    = 2in