Newton’s Cradle For Those Too Lazy To Procrastinate

Desk toys are perfect for when you don’t want to work. There’s a particularly old desk toy called the Newton’s cradle. If you don’t know the name, you’d still recognize the toy. It is some ball bearings suspended in midair on strings. If you pull back, say, two balls and let them swing to impact the other balls, the same number of balls on the other side will fly out. When they return, the same number will move on the other side and this repeats until friction wears it all down.

We think [JimRD] might be carried away on procrastination. You see, he not only has a Newton’s cradle, he has automated it with an Arduino. According to [Jim], this is his third attempt at doing so. You can see the current incarnation in the video, below.

There are two servos. One pulls back the balls and releases them and the other stops the balls in anticipation of the next operation. The servo that pulls the balls back is clearly magnetic. At first, we thought it was an electromagnet and that deenergizing it released the balls. That’s not the case. Instead, the servo arm has a permanent magnet, but foam decouples it from the ball so that if the arm pulls far enough away, the ball can escape.

Because of the differences in magnets, ball bearings, and other factors, if you try to duplicate this, you’ll probably have to experiment a little with the angles and speeds in the code. The ball stop servo is probably unnecessary, as long as you don’t mind waiting for the thing to wind down on its own.

If you don’t have a cradle, you could always make one yourself. We’d probably avoid using light bulbs, though.

11 thoughts on “Newton’s Cradle For Those Too Lazy To Procrastinate

    1. No, that doesn’t happen. I had the same thought that the magnet would magnetize the balls and play havoc with the swing but I don’t see any adverse effects happening. I have even left the first ball attached to the magnet over night and the next day it clacks away merrily. There is a slight magnetizing of the first ball but the momentum easily overcomes any attraction it has for the second ball and if anything there is a positive effect in that it minimizes the small spaces that develop between the balls during their clacking which does create a kind of chaotic effect. I think because of the ball is not in direct contact with the magnet, it gets very little magnetizing itself. But if you have seen another project where this is a problem, I would love to see it if you wouldn’t mind positing a link. Thanks.

  1. Too bad that massively scaling it up doesn’t work. Mythbusters attempted that in a ship drydock. The best they got was the ball on the other end just wiggling a little. What they used for the balls damped too much of the impact energy instead of passing it though.

  2. Thanks for the writeup Al. Someone commented that it would be nice to use an IR sensor to sense when the balls stop moving and then go down and pick up the ball. Also your idea of using an electromagnet is a great idea because you could vary the power to pick up one or two or three or four balls. Another idea is to put another magnet/arm/servo on the other side and pick up balls from both sides simultaneously. Lots of stuff still could be done but probably won’t.

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