The animated image above shows a typical Newton's cradle; a series of identically sized metal balls suspended in a metal frame so that they are just touching each other at rest. Each ball is attached to the frame by two wires of equal length angled away from each other. This restricts the pendulums' movements to the same plane.
If one ball is pulled away and released, it falls and appears to come to a dead stop on hitting the other balls. The ball on the opposite side of the series appears to instantly acquire the speed of the first ball and swings in an arc that one would expect of the first ball.
The intermediate balls appear stationary. In fact, the cradle continues to work even if the intermediate balls are physically clamped still. This is also counter intuitive - transmitting motion without moving. What actually happens is that the first impact produces a shock wave that propagates through the intermediate balls.
The behaviour of a pendulum follows from the conservation of momentum and kinetic energy only in the case of two pendulums. Indeed, if there are r pendulums there are also r unknown velocities to be calculated from the initial conditions. An additional condition for the observed outcome is that a shock wave has to propagate without dispersion through the chain.
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"Oh Newton, release this apple from its earthly shackles and live to fight another day".
Because love... Love will tear us apart again.
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