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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. 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=kx • Where x is the extension or compression of the spring. • K is called the spring constant • For a spring, a graph of Force against extension will produce a straight line of gradient k which passes through the origin. Force F Gradient = k Extension x 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=kx K=F/X = 10 / 0.05 = 200 Nm-1