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Homework Help: Finding the moment of inertia and torque for a spinning ball

  1. Feb 2, 2010 #1
    1. The problem statement, all variables and given/known data
    A ball is going around in a circle of radius 5 m.
    It starts from rest, and goes through an angle change of 26 rad in 5 seconds.

    The ball weighs 0.3 kg.

    Find its moment of inertia, and find the torque about the origin.

    2. Relevant equations

    3. The attempt at a solution
    What I'm doing seems too simple, so I bet it's wrong...

    I thought moment of inertia was just mass times radius2. My textbook doesn't give it for this situation but it gives the moment of inertia for other shapes, all of which are this but multiplied by some factor.

    And I think the torque would be zero because the force acting on the ball would be towards the center of the circle, and since torque is r x F, and F is 180 to r, rFsin(180) = 0.
  2. jcsd
  3. Feb 2, 2010 #2


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    Homework Helper

    Well it is just a point mass so the inertia is just mr2. The torque would be equal to the moment of inertia*angular acceleration.
  4. Feb 2, 2010 #3
    Why is the torque equal to the angular acceleration though?
  5. Feb 2, 2010 #4


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    Torque is the rate of change of angular momentum.

    [tex]\tau = \frac{d}{dt}(I \omega)= I \frac{d\omega}{dt}=I \alpha[/tex]
  6. Feb 2, 2010 #5
    Oh okay, thank you.

    This made me wonder if you can find the radial component of the force on the ball using torque, since torque is the rotational equivalent of force.

    I was going to find the radial acceleration, but I don't know how seeing as the radial acceleration is equal to ω2r, and ω is constantly changing!
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