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  1. [Newton’s Cradle](, commonly known as an office-desk toy, is a nice example of both momentum and energy conservation. It consists of several metal balls with identical mass m, that are suspended such that they just touch each other at rest. If l balls at the left side are displaced and hit the remaining balls with a speed v*_l_*, r balls at the left side will be ‘kicked’ with velocity v*_r_*. Momentum and energy conservation then reads:

    lmv*_l_* = rmv*_r_*

    1/2 lmv*_l_*^2 = 1/2 rmv*_r_*

    Combining both equations immediately yields l = r and v*_r_* = v*_l_*. Thus the same number of balls will be ‘kicked’ in each cycle.

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