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Newton's cradle. This device consists of a number of steel balls suspended from a frame in a horizontal line. When three of the balls are pulled back and allowed to swing, they impact the two stationary balls. The momentum is transferred along the line and conserved, causing three balls to move upwards to a similar height on the opposite side. 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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