**Power **is defined as the time rate at which work is done or energy is transferred.

The average power of a force is defined as the ratio of the work, *W*, to the total time *t *taken

The instantaneous power is defined as the limiting value of the average power as time interval approaches zero,

The work dW done by a force F for a displacement d**r **is dW = **F.**d**r**. The instantaneous power can also be expressed as

where **v **is the instantaneous velocity when the force is **F**.

Power, like work and energy, is a scalar quantity. Its dimensions are [M L^{2} T^{-3}]. In the SI, its unit is called a watt (W). The watt is 1 J s^{-1}.

The unit of power is named after James Watt, one of the innovators of the steam engine in the eighteenth century.

There is another unit of power, namely the horse-power (hp)

1 hp *= *746 W