What it shows:
Demonstration of elastic collisions between metal balls to show conservation of momentum and energy.
How it works:
Newton's Cradle (less affectionately known as Newton's Balls) consists of six rigid balls hanging in a row with bifilar suspension. The balls hang so that they just barely touch their neighbor.
Various initial conditions can be employed. A single ball displaced will collide with the remaining four, sending the ball at the far end off. Same idea for two or three balls. Four balls, and only the first two will stop; the center two will part of the reaction. You get the idea.
Newton's Balls in action
The original Newton's cradle (of balls) was devised over four hundred years ago by the distinguished French physicist, Abbé Mariotte, one of the ablest and most successful investigators of his time. Today these cradles are available commercially (usually as executive's toys); ours is from Sergent-Welch (0706), which has 1" diameter balls that are big enough to be seen from the back of the hall. Good accompaniment for the air track Elastic and Inelastic Collisions.
J.A. Zahm, Sound and Music, (A.C. McClurg & Co., Chicago, 1892).