1. Limited time only! Sign up for a free 30min personal 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!

Combined linear and rotational motion question

  1. Apr 25, 2014 #1
    1. The problem statement, all variables and given/known data

    A small solid disk (r<<R), mass m = 9.3 g, rolls on its edge without skidding on the track shown, which has a circular section with radius R = 9.7 cm. The initial height of the disk above the bottom of the track is h = 30.8 cm. When the ball reaches the top of the circular region, what is the magnitude of the force it exerts on the track? (Hint: how fast is it going then?)

    2. Relevant equations

    I = 1/2MR^2

    F(centripetal) = (mv^2)/r

    3. The attempt at a solution

    So I'm pretty sure you have to first calculate the velocity of the disk as it enters the circular part. However, I'm confused as to how as we are not provided with a time. Can you assume it is 1 second?

    Then the net force must equal the centripetal force at the top of the loop, which will probably be close to zero.
    And the speed of the object must match the centripetal force provided by gravity.

    so making the centripital force equal to mg gives you

    v= sqrt(rg)

    I've gotten this far, but I have no idea where to go from here. Please help!

    Attached Files:

  2. jcsd
  3. Apr 25, 2014 #2
    You can, but why do you need to? You need the velocity at the top of the circular track, not at its bottom.

    This assumption is not based anything substantial, and so best avoided.

    Then you can already answer the question in the problem: zero. Does that look right to you?
  4. Apr 25, 2014 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Use a conservation law.
  5. Apr 25, 2014 #4
    this problem is really tricky, I am having many problems trying to solve it
    Last edited: Apr 25, 2014
  6. Apr 25, 2014 #5
  7. Apr 25, 2014 #6
    what I did was saying that mg(h-2r)=1/2mv^2 at the top of the circle. would this be correct?
  8. Apr 25, 2014 #7


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

  9. Apr 26, 2014 #8
    Since ##r## is taken into account for potential energy, perhaps the kinetic energy due to rotation should also be taken into account?
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: Combined linear and rotational motion question