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What is Newton's Cradle?

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  • Written By: Niki Foster
  • Edited By: Bronwyn Harris
  • Last Modified Date: 03 November 2016
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Newton's Cradle is a device used to demonstrate conservation of momentum and kinetic energy. It consists of a series of identical balls, usually five or seven, each attached by two strings of equal length to a frame and just touching each other. First manufactured in 1967, this device is a popular desk toy. The device is so named because it demonstrates laws of physics discussed in Sir Isaac Newton's 1687 work, Philosophiæ Naturalis Principia Mathematica. However, 17th century French physicist Abbé Mariotte was the first to demonstrate the law of impact between bodies that governs the movement of a Newton's Cradle.

To use this device, the first ball on one end of the device is picked up and dropped. It transfers energy through the middle balls before causing the ball on the far end to swing up. When the ball descends, the action repeats. The middle balls do not apparently move, while the two on the ends clack up and down. A similar effect can be seen if two or three balls are picked up and dropped; the same number of balls on the other end will swing up and down, and any balls remaining in the middle will remain stationary.

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Newton's Cradle is popularly used in physics classrooms to show that kinetic energy and momentum are conserved in collisions. While this is a simplistic explanation, and the uniform balls and cables and restricted movement in the cradle make it a special case, not always applicable to real-world situations, the toy provides a helpful visual to students and can make science fun.

The original design of the Newton's Cradle, marketed by actor Simon Prebble, used steel balls on a wooden frame. Later, a sleek chrome frame became the common design. The world's largest Newton's Cradle, designed by Chris Boden, is on public display in Kalamazoo, Michigan. It is made of seven 15-pound (6.8 kg) bowling balls suspended from the ceiling by 20-foot (6.1 m) cables and is regularly used for scientific demonstrations.

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coling
Post 3

A more interesting and significant question about Newton's Cradle is the following:

For a three ball Newton's Cradle, when one ball is pulled aside and released, why do you never see that ball bounce back with one third of its original speed and the other two move off together in the opposite direction with two thirds of the speed of the original moving ball?

For such a cradle there are many solutions like that above which conserve momentum and kinetic energy but are never observed. For a full discussion of the behavior of Newton's Cradle see the article in "Science & Education", Volume 15, 2006, 597-617.

anon31326
Post 2

What are the basic energy changes taking place in this system?

anon31325
Post 1

What has occurred in terms of forces?

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