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Action Reaction on a discus thrower.

  1. Oct 18, 2013 #1
    Hi there,
    I am trying to understand the physics acting on a discus thrower, namely at their feet when they throw a discuss.
    first the player spins to pick up momentum for the discus, he or she does this by pushing on the ground but when the actual discus leaves the hand, is there an action reaction? isn't the discus just leaving their hand? if you were to simplify the problem to just a spinning arm on a pivot with a mass at the end, when the mass leaves the arm, does it push back on the arm, does the pivot feel the tangential force back?
    In other words if the discus arm was in space or on water, assuming you could spin it up, when you release it, would the arm be flung back? would the displacement of the pivot change? not in a circle but linearly?
  2. jcsd
  3. Oct 18, 2013 #2


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    Science Advisor
    Homework Helper

    For that simplified problem, there is no tangential force. But you have made the problem too simple. You don't just "let go" of the discus, you "throw" it.

    Think about some simpler situations first:

    1. What are the action reaction forces if you stand still, and throw the discus?

    2. What if you run forward in a straight line, and throw the discus while you are running?

    In the real discus throw, you "run" in a circle to increase your angular velocity before you throw.
  4. Oct 18, 2013 #3
    Thanks Sci Advisor. I am interested in this simplification.
    Suppose I had a platform in space, with this apparatus(spinning arm on a pivot and a mass on the end) bolted on to the platform, and that I put another similar apparatus above it spinning in the opposite direction so that the platform has no net rotation in space. Now if I release the masses so that they both travel in the same direction, will the platform move when the balls are released? is there a reaction that the platform experiences?
  5. Oct 18, 2013 #4
    when I say does the platform move, I mean a net movement, clearly the platform will oscillate while it is spinning.
  6. Oct 18, 2013 #5


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    Gold Member

    I dont think the spinning matters. When you "throw" from a platform in space, whether by fling, by compressed air, or by a spring, momentum is conserved. m*v of projectiles and m*v of platform are equal and opposite. The same basic thing is true for the discuss thrower in spite of all the pre and post throw motion.
  7. Oct 18, 2013 #6
    Thanks, so are you saying the platform will move back, or the momentum is conserved in the spinning, and the platform stays where it is?
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