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Moment of inertia rolling down a hill

  1. Oct 27, 2012 #1
    1. The problem statement, all variables and given/known data

    Two solid spheres -- a large, massive sphere and a small sphere with low mass -- are rolled down a hill. Which one reaches the bottom of the hill first?

    2. Relevant equations

    ICM= (2/5)MR2

    3. The attempt at a solution

    I thought that this would be the smaller sphere because it has a lower moment of inertia… but this isn't right. Can someone please explain?
     
  2. jcsd
  3. Oct 27, 2012 #2

    Simon Bridge

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    You have to do the physics ... what are the forces on the cylinders and the constraints?
    Have you just done a lesson or so on rolling without slipping?
     
  4. Oct 27, 2012 #3
    What physics do I have to do? My teacher talked about it a little… but honestly he is not very good at explaining things so I am having trouble understanding this.
     
  5. Oct 27, 2012 #4
    Just gravity is acting on them right? So the larger, more massive one would reach the bottom first?
     
  6. Oct 27, 2012 #5

    Simon Bridge

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    The wheel has to roll as well - so it will have an angular velocity.
    Have a look at how other's do it.
    http://faculty.wwu.edu/vawter/PhysicsNet/Topics/RotationalKinematics/RollingWithoutSlipping.html [Broken]
    https://www.physicsforums.com/showthread.php?t=169313
    ... there are also video tutorials on youtube.
     
    Last edited by a moderator: May 6, 2017
  7. Oct 28, 2012 #6
    okay so since the masses and radii drop out, they reach the bottom at the same time?
     
  8. Oct 28, 2012 #7

    Simon Bridge

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    why would the masses and the radii "drop out" - did you do any math?
     
  9. Oct 28, 2012 #8
    that's what it was on your second example… What math am I supposed to do??
     
  10. Oct 28, 2012 #9

    Simon Bridge

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    Have you had a look at the links I have given you?
     
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