A body is moved along a straight line by an engine which delivers a constant power. The distance moved by the body in time $t$ is proportional to

A body of mass 3 kg is moving under the action of a force which causes a displacement of $\left(t^3 / 3\right) \mathrm{m}$, where $t$ is time in seconds. The work done by the force in first 2 sec is

While a person climbs stairs, the gravitational potential energy of the person increases. The source of this energy is