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Q: I have an item called a “motion teaser.” It includes five heavy silver balls about an inch in diameter. Each is attached to a string and the strings are attached to a wooden frame. You swing one ball so it touches the next one and then they all swing back and forth. However, it stops in about a minute, Aren’t they supposed to keep swinging back and forth by themselves? Every once in a while, I see one of these in an old movie and the balls keep swinging back and forth indefinitely. Am I doing something wrong? I don’t see what the big deal is if you have to start it every other minute. Someone gave me this. I think this it’s from the 1970s or ’80s.

A: Your toy was invented in 1967 by Simon Prebble, an English actor, and is known as “Newton’s Cradle” because it demonstrates one of Isaac Newton’s laws of motion: “For every action, there is an equal and opposite reaction.” When you pull the first ball back and release it so that it swings and hits the row of balls, the energy is transferred through the line of balls to the ball on the other end, causing it to swing out at approximately the same distance and back to hit the stationary balls. If you pull two balls out, two balls will swing out from the opposite end. It’s not a perpetual motion machine because some momentum and energy are lost with each hit due to friction. The length of time it will keep going is based partly on how well it’s built. Toys like this were made under several names and in different sizes. They always have an odd number of balls, usually five or seven. Someone has even made Newton’s Cradle using 15-pound bowling balls hung from 20-foot cables.



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