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Pinball Physics, LIVE CATCH

  1. Feb 11, 2013 #1
    Hi guys,
    I'm trying to formalize the physics behind a real life phenomenon that happens in pinball, live catch:

    This video is not the best example, when you time it perfectly, the ball comes to a complete stop and just rolls down the cradle.

    When the ball hits a flipper that's held up, the ball just bounces as we would expect from an elastic collision (the flipper has rubber all around it).
    But if you hit the ball with the flipper EXACTLY when the flipper is at its apex, the ball stops completely.

    Now, since both momentum and KE are conserved (I know some energy is dissipated, but let's assume the ball-flipper is a perfect elastic system), why doesn't the ball bounce off even in a live catch?

    My explanation would be that for some reason the flipper is pushed down by the ball only when it's almost at the apex, and not when it's help up.
    Then the ball would be stopped by the flipper absorbing the downward acceleration due from the component of gravity parallel to the table, and the ball momentum as well.

    I believe flipper works with electro-magnetic energy, and I know that there is a switch that reduces the magnetic energy when the flipper reaches the apex. That would actually work against the above theory, since the flipper would be less resistant to being pushed down when it's held up.

    Is there anybody here that think he/she can shed some light on this?

    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Feb 11, 2013 #2


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    Hmmm. It looks to me like the following happens.

    The ball hits the flipper and begins to bounce off. Before it can do so, the flipper is released, which enables it to carry away the transferred energy/momentum from the ball. Before the ball can roll down any further, the flipper is engaged again comes back into contact with the ball while the flipper is at the top of its travel range, whatever mechanism holds it in place is where the flipper's energy and momentum go to instead of the ball.
  4. Feb 11, 2013 #3
    Thanks Drakkith,
    In a live catch there actually is no release of the flipper at contact (that's a drop catch).
    All you need to do is flip at the right time so that the ball connects when the flipper is at its apex, keep it pressed, and the ball stops.
  5. Feb 11, 2013 #4


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    Oh, I guess I didn't see the video correctly then.
  6. Feb 12, 2013 #5
    But it might be that the flipper, being at the end of its hinge joint limit, at the instant after the contact, bounces back from the limit. Then it would be exactly the same as you described.
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