Answers and Explanations

Review Questions

1. D
   The velocity vector points tangent to the circle in the direction that the ball is moving at the instant when it is at point A, which is toward the top of the page. The centripetal force and acceleration both point toward the center of the circle.
2. C
   Since the ball is moving toward the top of the page at the instant when it is at point A, it would move in that direction if the string were to be cut there.
3. D
   The period is
4. B
   
5. D

   The force holding the child on is the centripetal force:

   
6. A
   Torque is a vector and it is the product of force and lever arm length.
7. B
   The string must be hung a little closer to the 4 kg mass than the 3 kg mass so that the torque to the left of the string will equal the torque to the right of the string. We can write (3 kg)(4 length units) = (4 kg)(3 length units).
8. B
   According to conservation of angular momentum, if the radius is halved, the speed will double.