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Homework Help: Rolling ball down ramp

  1. Feb 28, 2005 #1
    In the figure below, a ball is let go from rest at the point shown on the ramp. The region on the left side of the ramp has friction; on the right side, the friction is negligible.


    Theres the picture. It won't upload cuz apparently its too big.

    A) What force causes the torque around the center of mass to make the ball roll while going down?

    B) Which direction is the friction force on the ball while on the left side of the ramp? (if no friction say zero)

    C) Which direction is friction force on the ball on the right side of the ramp? (if none say zero)

    (D) Is the ball still rolling without slipping on the right side of the ramp? If so, explain what force is giving the right torque about the center of mass to make it roll; if it is not rolling, then explain what is different between the left and right side that makes it not roll on the right side.

    I've gotten part way with what I think is right but any help would be appreciated, as I am still a bit foggy on rolling object concepts.

    For A, I put the force that causes the torque and thus causes rolling is the frictional force of the left side. The ball is let go and it starts to move and begins to roll due to friction opposing the forward motion. I do, however, have this suspicion that the force of gravity may in fact play a much bigger role because if let go, the force of gravity pulls down on the center of mass and causes a torque force.

    For B, I said that on the left side, the friction force is directed towards the right, this being due to the fact that the ball is moving right and wanting to slip to the left (spinning around the axis), and since friction opposes slipping the friction will be in the opposite direction of where the ball wants to slip, so friction is directed the same way as the ball's motion.

    For C, there is no friction due to the fact that friction can be neglected.

    For D, I said that the ball is now rolling while slipping because there is no friction to oppose the slipping. However, the question confuses me because it asks whether the ball is rolling without slipping or not rolling at all. Does this mean that the ball is just slipping but not rolling or what?
  2. jcsd
  3. Feb 28, 2005 #2
    Forget about your suspicion & go with your first answer. Since the net effect of gravity is acting AT the center of mass, it obviously can't exert a torque ABOUT the center of mass which is what would be needed to cause rotation. The rotation is caused by the friction, as you said.
    This contradicts your answer for part A, and is wrong. The ball is moving down the slope towards the right. Friction ALWAYS acts in the direction opposite to the motion. Therefore the friction is acting tangentially to the ball, up the slope towards the left, and this causes the ball to rotate clockwise.
    Correct. On the right side, NO force is causing it to rotate. Instead, it's rotating because there is no force to STOP its rotation. However, its translational motion (up the slope) is slowing due to gravity, and so it is rotating FASTER than it would if it were "rolling without slipping" (and in the absence of friction, it will continue to rotate clockwise at a constant angular velocity, even after it stops moving up the slope and starts slipping back down). So your answer, "rolling while slipping" is correct.
  4. Mar 1, 2005 #3
    thank you kindly my friend, that helped clear things up a lot
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