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Newton's Cradle office desk toy

  1. Nov 23, 2009 #1
    How does this work? I know energy is roughly conserved. But why, for instance, doesn't the one metal ball bounce back off the others when it hits, and all the other balls move slightly in the other direction? Is it because the balls are separated by a thin gap?

    http://www.giftmonger.com/acatalog/newtons-cradle2.jpg [Broken]
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 23, 2009 #2


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    Not only energy but also momentum. Taken together it follows: If a moving mass elastically hits an identical resting mass, it transfers all momentum to it.
  4. Nov 23, 2009 #3
  5. Nov 23, 2009 #4
    A.T. and TurtleMeister:

    Thank you. Sorry I forgot to mention in the OP that I know about elastic collisions. I understand that, at least during the collision, momentum is conserved along with energy. But then the strings pull the balls into a circular path.

    In the picture there are five balls. If you pull one back and let it smash into the other four, it transfers all its momentum and energy, and the ball on the other end pops off with that same momentum. But this would not happen if we replaced the four identical balls with one ball which has four times the mass. The first ball would bounce back off the heavy ball without transferring all its energy.

    I'm wondering if there are small gaps between the five balls or if they are touching when they hang at rest. If there are gaps, that would at least explain what happens when you pull back one ball. It slams into the second ball (the first one at rest), transferring all its momentum and energy, which in turn slams into the third ball, etc. But what about when you pull back two balls and release them? Would you expect two balls to transfer all their energy and momentum to the first ball at rest. That would seem to violate conservation of momentum and energy. But I think that in reality the two balls do transfer all their momentum and energy to the remaining three balls (whether or not it is transferred to each one successively, I don't know) . And somehow two balls pop off the other end with the same momentum the incoming two had.

    I know this satisfies the conditions of conservation of momentum and energy, but it doesn't seem like the only possible result that could satisfy those conditions. So how is the momentum and energy communicated from one end to the other?
  6. Nov 23, 2009 #5


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    Not sure if the gaps are needed. Video of a large scale model, here you can see that the ball does bounce back a bit, and that it's not perfect, with gaps bewteen some balls and not the others.

  7. Nov 24, 2009 #6
    It makes no difference whether the balls are touching or not. As long as each ball is able to freely rebound off the other balls, the action will be observed.
    Here's another link that addresses that question.

    Last edited: Nov 24, 2009
  8. Nov 24, 2009 #7


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    You can see it as happening successively. The inner of the two pulled balls, kicks off the most outer ball on the other side. Then the outer pulled ball impacts and kicks of the second ball on the other side.
  9. Nov 24, 2009 #8
    That is a fantastic link. It is as if the author read my mind and knew what questions I had. The rest of that website looks interesting, too. Thanks!

    That is a very cool demonstration with the "duck pin" bowling balls. thank you.
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