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