Newton's cradle. This device consists of a number of steel balls suspended from a frame in a horizontal line. When one of the balls is pulled back and allowed to swing, it impacts the stationary line of balls. The momentum is transferred along the line, causing the ball on the opposite end to move upwards to a similar height. This sequence of impacts continues until all the initial energy has been dissipated - through sound and heat in the collisions and in torsional friction in the suspending threads. Often used as a desk toy, this apparatus demonstrates conservation of momentum and energy.
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