Newton's Cradle
The Newton's Cradle is a classic demonstration of the law of conservation of energy and momentum.
Traditionally it consisted of five steel balls rolling on a cycloid curve but
a better version can be constructed by suspending the balls from a sturdy frame to minimise losses
through friction. Although the cycloidal path is lost in doing this, the advantages in avoiding
dynamical losses outweigh this consideration.
A large Newton's cradle can be constructed relatively cheaply using pool or snooker balls and a
wooden frame. The wires can be made from guitar strings, or a fine cotton cord.
Snooker balls are made from a plastic that is easily drilled and tapped. As shown in the
diagram above, the balls can be drilled and tapped to accept M4 threads. Some M4 bolts
can then be modified to provide wire break-off points. The diagram shows the wires suspending
the balls above the centre of percussion, although if possible it is more desirable to have the
break-off points lower down on either side of the ball closer to the centre of percussion. The wires are kept in place by simply
knotting them. This makes them easier to adjust in the inevitable event of the balls not sharing the
same level. The wires are chosen so that energy loss through violin modes is minimised.
For this, thin steel wires are best, like guitar strings (i.e. about 10 thou or so diameter).
We have built a cradle with five balls which works well. The size of the exhibit is important when doing class
or public demonstrations where the physical laws can be illustrated to a large group of people.