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Homework Help: Motion of a sphere

  1. Apr 25, 2007 #1
    1. The problem statement, all variables and given/known data
    A sphere is resting on a horizontal surface. A force parallel to the surface passing through the center of the sphere is applied.

    Describe the motion of the sphere
    1. Without any friction
    2. With friction.

    3. The attempt at a solution

    When there's no friction, then the motion is purely translatory. But, if there is friction, it will apply a torque which will force rotation as well as translation. The second part isnt right though. Why?
  2. jcsd
  3. Apr 25, 2007 #2


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    Homework Helper

    It cannot apply any torque, because the force goes right through the center of the sphere. What happens is that the friction touching the ball below is the culprit in creating this invisible torque.
  4. Apr 25, 2007 #3
    The force initially applied does not have any torques but it is the friction itself that acts as a torque that rotates the ball equivalent to (frictional force*radius of the sphere)
  5. Apr 25, 2007 #4
    Yeah, thats what I thought, but the book says that there can be no rotational motion under any conditions. I thought the book was wrong but maybe I'd missed something.
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