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Homework Help: Centripetal Force behind Rotation?

  1. Aug 18, 2007 #1
    What is the centripetal foce for the rotation on one's own axis?

    Take an example of a ball, you spinned it and left it in an isolated condition.
    It keeps on spinning. While each of its atom is doing circular motion around a fixed point. What is the centripetal force causing it to circulate and how can it last long forever?
  2. jcsd
  3. Aug 18, 2007 #2
    first it is u who provides the torque to start the rotaition.then it is the very same internal force that provides the centripetal accelaration which would have made each 'atom' move in a striaght line had u pushed the ball
    alsoassuming no friction,no external torque.... it would keep on doing so.
  4. Aug 18, 2007 #3
    How this internal force is such that it acts perpendicular on every atom. Is it the internal tension that is caused by mutual attraction of the atoms of the ball?
  5. Aug 18, 2007 #4

    Doc Al

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    Inter-molecular forces hold the ball together. When the ball is not spinning, the net force on any segment of the ball is zero. But when you spin the ball, tension is created. The outer layer of the ball is pulled inward by the next inner layer due to the increased inter-molecular forces. (Of course, if you spin the ball too fast the inter-molecular forces will not be strong enough to produce the required centripetal force--the ball will fall apart.)

    As long as no dissipative forces act, the spinning can continue. You've given it all the energy it needs. (Maintaining the tension does not consume energy.)
  6. Aug 18, 2007 #5
    well philosophically speaking i didn't like ur question
    the reason being since the ball is spinning and since each atom is accelarating in a direction perpendicular to it's speed and since force produces accelaration and since the only 'source' of force is the atoms within so i gave the answer.
    had the ball been moving linearly it would have been the same internal force that i would have said provides the necessary linear accelaration
  7. Aug 18, 2007 #6
    The centripetal force is whatever holds the ball together (forces between the molecules)

    Theoretically it can last forever if there are absolutely no other forces on it.
  8. Aug 18, 2007 #7


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    But when the contact exetrnal force is released, the ball (lets say it is a block to avoid the spin component) would move linearly at constant velocity and without linear acceleration or any internal forces acting...
  9. Aug 18, 2007 #8
    if the internal forces wouldn't have acted hoe would an 'atom' deep inside where u don't directly apply the force ever move
  10. Aug 20, 2007 #9
    i.e. why the atoms deep inside move at lower speeds and ultimately the center of mass doesn't move at all in pure rotation.
  11. Aug 20, 2007 #10
    well first f all i was talking in general motion.
    secondly it doesnt matter whether the atoms were moving faster or slower what's important is they were at rest and then started moving
  12. Aug 20, 2007 #11


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    What do you mean by "acts perpendicular on every atom". What do you mean by "perpendicular" to an atom?

    You apply force, in whatever, way to some atoms on the outside of the ball. They each, through the magnetic-electric forces between the atoms, the very forces holding the ball together, pull the rest of the atoms around with them. As far as why some are moving slower so that the ball goes around its axis rather than moving away is concerned- you were the one that said this initial force WOULD make it move that way. Obviously, that depends on exactly how the force is applied. Most ways of applying a force would make the ball both rotate and move away. YOU need to specify exactly how the force is applied before anyone can answer your question precisely.
  13. Aug 20, 2007 #12
    An atom in a freely spinning ball can be considered to be going a circular motion, so, I only asked what is the centripetal force for this circular motion.
    Obviously, this centripetal force must act perpendicular to it's velocity at any time.
  14. Aug 20, 2007 #13


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    Each atom is in motion. If the atoms in the ball were not molecularly linked, each atom would follow a straight line in the direction of its last motion. i.e. the ball would fly apart. The intermolecular bonds that make up the rubber stop each atom from following this course.

    While all the intermolecular bonds pull in their own particular direction, the net force of these individual forces is towards the centre of mass of all atoms that are bound together. That is the source of the centripetal force. And that is why it is, as you say, perpendicular (I think what you're describing is "perpendicular to the tangent" and the word that describes is "radial" i.e. radiating from the centre of the object.).
    Last edited: Aug 20, 2007
  15. Aug 21, 2007 #14
    Oh, yes I exactly meant what you said, thanks for reply.
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