1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Static friction for a ball rolling without slipping

  1. Jan 10, 2015 #1
    1. The problem statement, all variables and given/known data

    A bowling ball rolls without slipping up a ramp that slopes upward at an angle β to the horizontal. Treat the ball as a uniform solid sphere, ignoring the finger holes. Explain why the friction force must be directed uphill.

    2. Relevant equations

    F=ma, torque=I(alpha), a=r(alpha) (I don't think I need any of these for this question)


    3. The attempt at a solution

    I thought that because the instantaneous velocity for the contact point is in the downhill direction, the friction force must act uphill. However there's an example in my textbook in which the ball rolls downhill and the force of static friction is still uphill. Apparently, the force of friction acts uphill wether the ball is rolling uphill or downhill.
     
  2. jcsd
  3. Jan 10, 2015 #2

    Nathanael

    User Avatar
    Homework Helper

    This would be correct if the friction was kinetic friction. But, if the ball rolls, then the contact point does not slide (the instantaneous velocity of the contact point is zero!) and so the friction is static, which means it is free to act in either direction.
     
  4. Jan 10, 2015 #3
    Ok, so how do I know it's uphill in this case? I think it's because gravity acts downhill and the force of static friction has to oppose gravity.
     
  5. Jan 10, 2015 #4

    Nathanael

    User Avatar
    Homework Helper

    It can act in either direction that it needs to. The key is that the ball is rolling the whole time. What does it mean for the ball to be rolling?
     
  6. Jan 10, 2015 #5
    That it's accelerating? I'm not sure what you mean.
     
  7. Jan 10, 2015 #6

    Nathanael

    User Avatar
    Homework Helper

    Rolling means there is a special relationship between the angular velocity and the linear velocity.
    Look at your "relevant equation"
    or equivalently, [itex]v=r\omega[/itex] where v is the linear velocity and ω is the angular velocity.
     
  8. Jan 10, 2015 #7
    If the linear velocity is decreasing (it should be due to gravity), the rotational velocity also decreases. So static friction needs to act uphill to slow the ball down?
     
  9. Jan 10, 2015 #8

    Nathanael

    User Avatar
    Homework Helper

    Exactly :)
    If the linear velocity is decreasing, then the torque (from friction) must slow the ball's rotation.
     
  10. Jan 10, 2015 #9
    This topic makes so much more sense now. Thank you for your help! :)
     
  11. Jan 10, 2015 #10

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    To clarify one point:
    In rolling contact with a stationary surface, the point of contact of the wheel/ball is instantaneously stationary.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted