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Effect on rolling motion after an inelastic collision.

  1. Mar 30, 2012 #1

    I have a problem which I can't figure out.:confused:

    Let us take a sphere which is rolling purely at a constant velocity vo.
    Now, if the sphere were to collide inelastically with a wall, with coeff. of restitution = e.
    Then what is the time after which the sphere starts pure rolling again?
    Given coeff, friction = μ

    I went about by considering that after the collision, the particle will have evo velocity, but the same angular velocity ω=vo/r.

    Now, since the sphere is translating, the angular velocity is in the opp. direction. So the frictional force will cause a torque to change ω.

    So, if the body was initially moving rightwards, with clockwise ω, it's new velocity will be leftwards, but ω will remain clockwise. So the frictional force must act towards right to apply a counter-clockwise torque.

    After this, I couldn't figure out which way to go.:uhh:

    Any help appreciated.!:approve:
  2. jcsd
  3. Mar 30, 2012 #2


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    Science Advisor

    Usually the collision will change the angular velocity as well. If the ball rolls the force of the wall on the ball is not horizontal. It is slightly upwards generating a torque.

    But can you assume zero friction with the wall to avoid that. Then the ball will slide on the ground until the friction makes it roll. Use the torque from friction and moment of inertia to find the angular acceleration. Keep in mind that the friction also reduces the velocity, while it changes the angular velocity.
  4. Mar 30, 2012 #3
    So, taking the torque about the COM and then finding the angular acceleration, what happens to the linear velocity? I think the friction force will act in it's opposite direction and stop it from sliding.
    so that a = f/m.

    Then, I cud equate it as -

    v-at = r(ω - αt)
    and get the time?
  5. Mar 30, 2012 #4
    thx, just worked it out. got the right answer.

    Anyway, I was thinking that if the wall were shorter than the sphere itself? lets say that the wall has a height h, where h < r, i.e., the wall has a height less than that of the height of the center of the sphere. SO, only a corner of that wall will touch that sphere.
    Let the friction of the wall also be high enough that it resists slipping of the sphere upon contact. Then what?
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