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!

A rolling ball

  1. Jan 10, 2013 #1
    1. The problem statement, all variables and given/known data
    Hi

    Im trying to solve this question, and I have the solution of it in front of me, but I really cant understand why its true.

    The question is this: A bowl with radius R is given, it cant move. In it there's a ball with radius r << R, and a mass m.
    Find... (lots of subclauses for the question)
    A picture is attached.

    My question is this:
    Am I mixing the directions of the forces/torques?

    F is the friction force. I think the torque equation should be I*w_dot=F*r.
    and the equation for the tangential direction should be ma=F-mgs*sin(theta).

    However, in the solution I see here the torque equation appears with a minus sign: I*w_dot=-F*r, which obviously changes the answers of the things I need to find.

    Should it be with a minus? Or without? Thank you.

    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Jan 10, 2013 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    This doesn't look right to me at all. If we take moments about the centre of the ball, there is indeed a torque Fr anticlockwise, and it does tend to reduce theta. But ##\ddot{\theta}## is not the angular acceleration of the ball.
    There should be another angle in the picture - the angle through which the ball has rotated. It is related to theta by a simple formula.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: A rolling ball
  1. Rolling ball (Replies: 2)

  2. Rolling ball (Replies: 3)

  3. Ball Rolling (Replies: 5)

  4. Rolling Ball (Replies: 13)

Loading...