There is a popular toy consisting of five steel balls, all of the same size and mass, hanging side by side in a row (many people keep them on their desks). Pull out the end ball and drop it against the row, and one pops of the other end. If two balls are pulled aside and dropped together, two pop out from the other end. The balls seem to count! How do they accomplish this trick?

Friar Tuck

This ia called a Newtons Cradle The gracefully swinging and clacking-colliding balls, demonstrate Newtons Third Law of Motion: that for every action force there is an equal and opposite reaction force. Nov 07 02, 7:32 AM

Barrow boy

The Newton's Cradle demonstrates the principle of conservation of energy. When one ball is swung into the other four, nearly all the kinetic energy is transferred through all the others to the ball on the far side which then swings out away from the others. When TWO balls are swung out the energy is doubled. When that doubled energy is transferred through the other balls, it is large enough that it can propel both balls at the far end. The whole thing works because each ball weighs the same as its fellows. If you used balls of different weights all sorts of funny interactions would occur. Nov 07 02, 11:32 AM