A student carries a heavy bag up a flight of stairs at a steady speed, then walks along a horizontal corridor at the same steady speed. State what happens to the rate at which the student does work against gravity as they move from the stairs to the corridor, and explain why.

The rate of doing work against gravity decreases to zero when the student moves onto the corridor.
1. On the stairs the student and bag gain height; the force of gravity acts downwards while the student moves upwards, so work is done against gravity.
2. On the corridor the motion is horizontal, so there is no change in height and the displacement is perpendicular to the gravitational force.
3. Work is only done when a force has a component in the direction of the displacement; here that component is zero.
4. Therefore the power (rate of doing work) against gravity falls to zero on the corridor.