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Homework Help: Hollow Rolling Sphere up incline

  1. Nov 17, 2011 #1
    1. The problem statement, all variables and given/known data
    An 800 g, 40.0 cm diameter hollow sphere is rolling along at 4 m/s when it comes to a 25 degree incline. Ignoring any friction, how far along the incline does it roll before it stops and reverses its direction?

    2. Relevant equations
    rolling momentum
    motion equations

    3. The attempt at a solution
    i have no idea where to start
  2. jcsd
  3. Nov 17, 2011 #2
    Re: Hollow Rlling Sphere up incline


    Moment of inertia of a hollow sphere is I = 2/3 mr2 (according to Wikipedia).

    The kinetic energy of a system is the sum of its translational and rotational motion.

    E = 1/2 mv2 + 1/2 Iω2

    Of course, this has to be equal to the energy lost from going up that incline, which is mgh.

    Also note that there is the assumption that the sphere is rolling without slipping.

    If you're studying calculus and you run into problems, take the derivative. If you're studying linear algebra and you run into problems, row-reduce. When you're studying kinetics and you run into problems, examine the energy of the system.
    Last edited: Nov 17, 2011
  4. Nov 17, 2011 #3
    Re: Hollow Rlling Sphere up incline

    I did this and got E=10.66, now do i set that equal to mgh to find height?
  5. Nov 17, 2011 #4
    Re: Hollow Rlling Sphere up incline

    It's rolling UP an incline. Go figure.

    And fyi, you should never calculate for raw numbers until the very end of a problem. It's bad practice.
  6. Nov 17, 2011 #5
    okay, but how do i account for the incline?
  7. Nov 17, 2011 #6
    [PLAIN]http://img194.imageshack.us/img194/2649/220pxtrigonometrytriang.png [Broken]

    Last edited by a moderator: May 5, 2017
  8. Nov 17, 2011 #7
    i know what a triangle looks like but i dont know how to deal with energy and an incline
  9. Nov 17, 2011 #8
    mg is a force. h is a height. Forces are vectors. You can put them on triangles.

    I recommend reviewing energy in a system that only involves translational motion. This is a pretty basic concept and if you don't understand it, you shouldn't be studying rotational motion.
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