Consider a waterwheel in a pond. The falling water from the pipe exerts a force on the waterwheel making an angle Φ with the position vector, then the work done by the force to rotate the wheel through a small angle, dθ, is FsinΦds. Since the arc ds is equal to r times dθ, the work done is the product of FsinΦ and rdθ. Recall that the magnitude of torque equals rFsinΦ. Therefore, on substituting, the expression for work done equals to τ times dθ. In general, if a rigid body rotates from θ1 to θ2, the total work done on the body equals to the integration of the product of net torque and angular displacement. The instantaneous power delivered by a force is the rate at which the work is done by the force to rotate an object about its fixed axis having a constant torque. Therefore, power is expressed as τ times dθ/dt. Since dθ/dt is the angular velocity ω of the wheel, power equals to torque times angular velocity.