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Work, Energy And Power

Question
CBSEENPH11017161

Derive an expression for potential energy stored in spring.

Solution
Consider, a massless spring attached with mass m at one end and the other end of spring be connected with a rigid wall.
When we pull the mass towards C, the restoring force directed towards A is set up in spring. Work has to be done against this restoring force in order to displace the mass and hence this work is stored in the form of P.E in the spring. 
Let at any instant, mass m be at a distance x from A. 
Restoring force at this instance is, 
                      straight F with rightwards harpoon with barb upwards on top subscript r space equals space minus space k x space
Therefore, to keep the mas in equilibrium, we have to apply the force Fa equal and opposite to -Fr.
If the mass is further displaced by dx with rightwards harpoon with barb upwards on top, the amount of work done dW for this dispalcement by applied force is,
dW space equals space straight F with rightwards harpoon with barb upwards on top subscript straight a space dx with rightwards harpoon with barb upwards on top space

space space space space space space equals space straight F subscript straight a space dx space

space space space space space space equals space straight k space straight x space dx

Total amount of work done in order to displace the mass from mean position of A to C is using applied force Fa is, 
straight W space equals space integral subscript straight A superscript straight C dW space equals space integral subscript 0 superscript straight a straight k space straight x space dx space equals space 1 half kx squared vertical line subscript 0 superscript straight a space equals space 1 half ka squared space
Work done by applied force against this restoring force is stored in the form of potential energy is, 
                straight W space equals space straight P space equals space 1 half ka squared