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Angular Acceleration and Gravity

  1. Nov 26, 2008 #1
    Hello all,

    The other day I observed an occurrence in my Physics class which I do not understand. The teacher set up a rotating platform with a second platform attached to it, so it made a 45 degree angle with the rotating platform.

    He then proceeded to put a ball at the top of the second platform, and spun the whole thing and the balls did not fall for quite some time ~30 seconds.

    How come the ball doesn't immediately roll down the elevated platform? I have thought about this and don't really get it.

  2. jcsd
  3. Nov 27, 2008 #2


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    The ball on the second platform will experience a force which will try to increase its raduis of rotation if the angular velocity is high enough to overcome the downward force of the earth's gravity.
    As the rotating platform loses velocity (due to frictional forces) the centrifugal force on the ball will also fall accordingly until it is less than the downward pull of gravity. After that the ball will roll down the incline. The key to keeping the ball 'suspended' is the velocity of rotation of the platform.
    If you have a children's round-about in your local park stand on its perimeter while someone spins it round quickly and you will physically feel the force on your body and you will be compelled to lean toward the axis of rotation (in the centre). As the rotation slows you will feel the force on your body reduce accordingly, and your angle of lean will change.
    The same thing happens to the ball on the platform.
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