This demonstration consists of a pair of DC motors mounted on a piece of MDF with wheels so that they can be used to propel a loop of string. A pair of batteries provides their power.

This device shoots out a loop of string in a continuous ellipse. With it you can show centripetal force and slow moving waves; if the loop is slightly disturbed, slow moving waves will travel along the string.

• String shooter

Simply flick the switch. Make sure that the string isn't tangled up or caught on anything before you do.

You may have to repair a string breakage. It isn't enough to just tie the broken ends of the string back together, since any knots in the string will get caught between the wheels. A needle and thread can be used to more cleanly splice the string back together.

The rubber on the wheels may also wear out. Currently it's made out of pieces of rubber balloon stretched over the wheels.