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!

Velocity of a Sphere Rolling in a Circular Bowl

  1. May 16, 2015 #1
    1. The problem statement, all variables and given/known data

    A solid sphere of mass M and radius a is released at vertical height y=R and rolls down a circular bowl without slipping, find an expression for the velocity of the sphere's center of mass at the bottom of the bowl.

    2. Relevant equations


    ##I=I_c+Md^2##
    [itex]I=\frac {2} {5} Ma^2[/itex]
    ##U=Mgh##
    [itex]E_r=\frac {1} {2}I\omega^2[/itex]
    [itex]E_t=\frac {1} {2}mv^2[/itex]



    3. The attempt at a solution

    The sphere is released from height R, so it's total energy is given by [itex]E=mg(R-a)[/itex] (as the radius of the sphere is non-negligible).
    Taking the bottom of the bowl to be the zero of potential energy;


    ##E_T=mg(R-a)=\frac {1} {2} I\omega^2+\frac {1} {2}mv^2##

    At the bottom of the bowl, all gravitational potential energy will have been transformed into translational and rotational kinetic energy.
    As the sphere is in rotation around it's contact point, via the parallel axis theorem:
    [itex] I=I_c+Md^2 [/itex]
    ## I=\frac {2} {5} Ma^2+Ma^2##
    ## I=\frac {7} {5} Ma^2 ##


    And as ## \omega^2=\frac {v^2} {r^2}, ##

    ## E_T=mg(R-a)=\frac {1} {2}mv^2+\frac {7} {10} ma^2 (\frac {v^2} {r^2}) ##

    Following this through I come up with ##v=\sqrt{\frac {5g(R-a)}{6}}##

    Rather than the (correct), ##v=\sqrt{\frac {10(R-a)}{7}}##

    I know if I only count the rotational kinetic energy I end up with this expression, but I don't understand why I wouldn't also count the translational kinetic energy?
     
    Last edited: May 16, 2015
  2. jcsd
  3. May 16, 2015 #2
    Sorry, I accidentally hit enter half way through writing this
     
  4. May 16, 2015 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    You've effectively counted the linear KE twice. Either take the sphere's motion as a linear horizontal speed of its mass centre, plus a rotation about the mass centre, or take it as a rotation about the point of contact with the bowl.
     
  5. May 16, 2015 #4
    Thank you so much! I think I understand a lot better now (that was a lot more painless than expected).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Velocity of a Sphere Rolling in a Circular Bowl
  1. Rolling Bowling Ball (Replies: 7)

Loading...