1. Energy stored in a Stretched String When stretching a rubber band or a spring, the more we stretch it the bigger the force we must apply.

2. Hooke’s Law For an ideal string we that to double the stretch we must double the force. Hooke’s Law states – For an ideal spring the extension or compression is proportional to the force producing it F = k x Where x is the extension or compression of the spring. K is called the spring constant

3. For a spring, a graph of Force against extension will produce a straight line of gradient k which passes through the origin. Force F Extension x Gradient = k

4. Example A spring of length 10cm is extended to 15cm when a 1kg mass hangs from it. Find k X = 15cm – 10cm = 5cm = 0.05m F = mg = 1kg x 10ms -2 = 10 N F = k x K = F / X = 10 / 0.05 = 200 Nm -1