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Pinball's "Live catch" defies physics!

  1. Sep 11, 2014 #1

    I have been using this skill for a few months now and I feel like it is magic. It's been driving me crazy how this works...

    I have only ever taken AP high school (non calc based) physics and one thing I remember is that for every reaction there is an equal and opposite reaction so the force of the flipper hitting the ball at the top of it's stroke shouldn't "cancel the momentum" of the ball isn't that jibberish?

    Here is what would happen if you didn't contact the ball at the exact moment the flipper is fully deployed either:

    a) you deploy the flipper too soon and it is fully deployed and stationary when the ball hits it and the ball will bounce away with too much speed and you often loose control and the ball may drain.


    b) you deploy the flipper too late and you strike the ball and shoot it up into the playfield likely not in a direction you have intended.


    c) you are so late that the ball hits the undeployed flipper and bounces away or onto the other flipper (this never happens if you are attempting a live catch)

    this is the explanation I read on a decent pinball tips website:

    Flip Trap (aka live catch)

    This technique is the opposite of the Drop Stop. Hit the ball so that it collides with the flipper just as the flipper reaches the point where the flipper is fully extended. If you hit the ball such that it meets the flipper just before it stops extending, the enery imparted by the flipper hit will cancel out the momentum the ball has and the ball will stop right on the flipper where you can trap it.

    If you guys can help me understand this it would be great. I'd appreciate a conceptual explanation not just mathematics. Thanks!
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Sep 12, 2014 #2


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    The collision between the ball and flipper is not that elastic, so on some pinball machines just having the flipper extended when the ball drops down on results in a small bounce to the side barrier and if the side barrier isn't one that propels the ball or if it doesn't trigger, the ball can be captured.

    If the flipper is triggered just before the ball collides, and there's some slop in the pivot axis of the flipper, it goes a bit past the driven range, and the collision drives the flipper back a bit, resulting in an even smaller bounce by the ball.
    Last edited: Sep 12, 2014
  4. Sep 12, 2014 #3
    Thanks. So what causes the difference between the effectiveness of a live catch verses just having the flipper held up and waiting for the ball? If you do a live catch right the ball will come to a complete stop which I've never seen happen when the flipper is just held up waiting for the ball.
  5. Sep 12, 2014 #4


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    Depends on the pinball machine, how much "slop" there is in the flipper, the weight of the flipper, how elastic or non-elastic collisions with the flipper are, ... . The live catch video always showed the ball bouncing as opposed to just stopping, so my guess is that it's not possible to just stop the ball on that pinball machine.

    I was a bit surprised by the comments in the video, where the guy states it changed the way he plays pinball, since live catch methods were figured out with early pinball machines, but there aren't many pinball arcades anymore, so there's no "community" to pass along this information, even if it was well known decades ago.
  6. Sep 12, 2014 #5
    Most of the skills I've learned I learned online because of the lack of pinball players these days. When I go to play pinball there are usually zero to two other people playing even at the best spots for pinball in the area. Thankfully my area had a pinball league I'm considering joining
    The live catch significantly reduces the bounce from the flipper usually allowing you to save the ball compared to just having the flipper raised.

    I don't understand why though what is the difference. Your explanations help me understand why the ball can slow down when hitting the rubber but I still don't understand the difference I see between the live catch and just trying to trap the ball.

    Is what causes a collision to be elastic or less elastic that the material compresses and redirects some of the force in directions other than the vector of the initial force? This has got me thinking about why when you bump jello and it shakes why does it ever stop shaking. Maybe it has to do with waves or friction on the microscopic level. I don't understand whether forces ever cancel each other out or the opposite principal that for every action there is an equal and opposite reaction. Physics really interests me and I wish I knew more about it.

    Oh also I have had rare live catches where the ball immediately stopped I just figure I need to get better at the skill for it to happen more regularly.
    Last edited: Sep 12, 2014
  7. Sep 12, 2014 #6
    Physically - I believe part of this action is based on the rotational inertia of the ball, it is rolling "into" the flipper and can not instantly change its rotation, some of the energy makes the ball pop up, but also the 1-3" that the ball does rebound it is still rotating, as it if is rolling down, as it lands the rotational inerta drives the ball back towards the flipper.
  8. Sep 12, 2014 #7
    That video isn't a really "good" live catch. I really good one actually does stop the ball then it rolls back to a cradle as opposed to a jump back to the cradle position.
  9. Sep 12, 2014 #8


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    There's some type of solenoid mechanism to drive the flipper, and some type of stop to limit how far forwards the flipper can rotate, but there's some slop in this, so the flipper (and whatever is directly attched to the flipper) momentum causes the flipper to move a bit forwards after reaching the stop. If the ball collides with the fliipper during this time, then it's just the flippers momentum involved during the collision. If the stop is soft and compressable, then the final portion of the flippers forward rotation involves decreased net torque, allowing it to absorb some of the momentum.
    Last edited: Sep 12, 2014
  10. Sep 12, 2014 #9
    These responses are very helpful but also bring up more questions and make me wonder which combination of factors fully explains the phenomenon. Here is what they are saying about it on pinside


    Here is a better video of a perfect live catch
    http://killscreendaily.com/media/uploads/live-catch-%5Bthe-addams-family%5D-tumblr.gif [Broken]
    Last edited by a moderator: May 6, 2017
  11. Sep 12, 2014 #10


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    One of the comments is that momentum is lost, it isn't lost, just transferred to something else (eventually the pinball machine and even the earth). Energy can be lost from a collision by converting some of it to heat such as compression of rubber. The thread did mention that some flipper rubbers are too elastic to do a live catch. The slop explanation is what I've heard before. The pool ball analogy is also flawed, since in a no spin straight on collision, the moving ball completely stops and the other ball moves away about the same speed as the original ball, since the collision is nearly elastic. Similarly for the flipper, if due to slop and some overshoot, the solenoid doesn't drive the flipper unless it's moved back to before it's normal solenoid driven position. Depending on the effective inertia of the flipper and whatever is attached to the flipper, combined with how elastic the collsion is, then stopping the ball and moving the flipper back a bit is one possible outcome. As mentioned previously, the rolling motion of the ball reduces the bounce.
  12. Sep 12, 2014 #11
    I'm a little tipsy so I'm going to save the thoughtful response for later. I am at a bar and experimented with the pool balls. One moving ball hitting another moving ball transfers the momentum into the other ball fully and due to my imperfect operation of this experiment haha the first ball may slowly roll off to the side at nearly a 90 degree angle to the original vector

    I was referring to two balls moving at the same speed directly toward each other along a line. In this case they bounce apart at the same rate
    Minus friction. So with the flipper it must absorb the energy from the ball by being pushed back. Elastic collisions seem to occur in an instant whereas inelastic collisions occur over a period of a fraction of a second and allow for deceleration. Correct?
    That seems irrelevant upon more consideration.
    Last edited: Sep 12, 2014
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