Inertia

Equipment Neded

1 block of wood
2 strong piece of string

The Experiment

Suspend a block of wood from the end of one piece of string.

Tie the other piece of string to the underside of the wood.

Pull slowly and steadily on the lower string, gradually building up the downward pressure and the upper string will break.

Give the lower string a sharp, powerful tug and the bottom string will snap.

When you tug sharply in this manner, the inertia of the wooden block prevents the force you have used from reaching the upper thread.