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Homework Help: Rotational Motion and Linear Speec

  1. Oct 20, 2008 #1
    1. The problem statement, all variables and given/known data

    A rubber ball with a radius of .048 m rolls along the horizontal surface of a table with a constant linear speed v. When the ball rolls off the edge of the table, it falls 0.85 m to the floor below. If the ball completes 0.82 revolutions during its fall, what was its linear speed, v?

    2. Relevant equations
    I know to find linear speed the equation v=rw must be used.
    Revolutions are changed to radians so it's 5.15 radians

    3. The attempt at a solution

    I tried to turn revolutions into radians and then use the equation T=2pi/w
    and then did v=rw. I don't really know where to begin this problem since the problem has revolutions and not rpm. When i solved it like rpm it didn't work.

    please help me!! I'm so confused!!!
  2. jcsd
  3. Oct 20, 2008 #2
    Well, you know how to convert rotational speed to linear speed using the equation in step 2 - all you need is the rotational speed and the radius of the ball. You also know the radius. So the question is, how do you find the rotational speed?

    You are given a distance the object falls (under acceleration from gravity), and you are given the number of rotations. Rotations / time is rotational speed. See if you can calculate the time and go from there.
  4. Oct 20, 2008 #3
    so to find time can i just divide distance/acceleration and then take the square root?
    Also when you say revolutions/ time = rotational speed I have to change revolutions to radians correct?
  5. Oct 20, 2008 #4
    never mind i figured it out! thanks so much for your help!
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