A popular demonstration for illustrating elastic collisions and the conservation of energy and momentum in the classroom is C7-11: Collisions of Balls of Equal Masses. Also popularly called Newton’s Cradle, as it helps us illustrate Newton’s laws of motion even if Newton himself may never have had one, you can find these in many places as entertaining desk toys; but they show us some important physics.

You can see the demonstration in action in our new video featuring Dave Buehrle.

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The simplest and most straightforward explanation for the behaviour of this device is just that – that it is an application of basic conservation laws. The collisions between these hard steel spheres are very nearly elastic, so nearly all of the momentum of the incoming spheres is transferred to the outgoing spheres, and nearly all the energy as well so they rise to the same height on the other side. A pendulum swinging back and forth is a classic illustration of the exchange between kinetic energy (from the velocity of the pendulum) and gravitational potential energy (the potential energy the stationary pendulum has as a result of its position when paused at the top of its swing). And this demonstration is, in a sense, just a set of pendula all swinging together, exchanging their energies and momenta, and we can simplify it be treating only the displaced balls as a single pendulum.