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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 = k x• 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.
ForceF
Extension x
Gradient = k
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
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