What Are Newton's Cradles?
Newton's cradle is a toy consisting of balls hanging as pendulums. When some balls on one side are released to collide with the others, the same number of balls release on the other side.
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Linear
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A basic characteristic of Newton's cradle is that the balls hang in a straight row. To maintain the linearity when swinging, the balls hang not from one line, as a regular pendulum, but from two.
In Contact
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Another basic characteristic is that its balls rest in contact. This allows propagation of the shock wave through the balls without dispersion.
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Size
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In commercially available Newton's cradles, the size of the balls is generally about one inch or less each, hanging from strings of less than half a foot.
Material
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The balls are usually made of steel, to enable inelastic collisions. An elastic collision, such as from colliding rubber balls, would not give the same result.
Two Equations
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When linearity, contact-at-rest and inelasticity are maintained, the only solution to energy and momentum conservation equations is for the same number of balls to leave one side as are incident on the other. The remaining balls stay still.
Other Names
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Newton's cradle is also known as "balance balls," because the incident and outgoing ball count is equal, and "executive ball clicker," because it is often used as an ornament on office desks and makes a clicking noise when the balls collide.
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