LAW OF CONSERVATION OF ENERGY                                                             GO BACK

The Law of Conservation of Energy states that Energy can neither be created nor destroy during transformation, it can only be converted from one form to another form.

A stone at height h possesses a potential energy of  mgh. this potential energy is converted to kinetic energy when the stone is released.  

Potential Energy + Kinetic Energy = constant

at the height h , v=0  then mgh= constant

when the stone is released and h=0, 

= constant

Example

 A stone of mass 3kg falls from height of 100m . What is the kinetic and potential energy at height 40m. What will be the velocity on reaching the ground ? take g=10

solution

At height 100m

PE + KE = constant

mgh + 0 =constant

3 x 10 x 100= constant

3000J = Constant

The potential energy at height 100m = 3000J 

The kinetic Energy at height 100m=0

at height 40m 

PE + KE = 3000

3 x 10 x 40 + KE= 3000

PE=1200J

KE=1800J

On reaching the ground

PE + KE= Constant

0 + KE = 3000

=3000

v= 44.72m/s

The total potential energy has been converted to kinetic energy on reaching the ground.

Energy Conversion in a Pendulum

When a pendulum oscillate from peak  h to center c and back to peak h, potential energy is being converted to kinetic energy.

At peak h the total energy is constant PE + KE = Constant while KE=0

At Center c the PE=0 therefore KE=constant

At any point in between h and c , PE+KE =constant, loss in PE= Gain in KE

The velocity v of the pendulum at any point in between c and h can be calculated from above as

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