If a man moves up an escalator at a constant speed, the average pressure which he exerts on the staircase will remain unaltered and equal to his weight. Therefore the force with which the motor must drive the staircase will also remain the same. However a mounting man will reach the top of the escalator sooner, and therefore the distance travelled by the escalator during the period of the man's climb will be less than when the man is stationary. Therefore the work done by the motor of the
WORK, POWER, ENERGY 131
escalator in raising a moving man will be less than that done in raising a stationary one (the other part of the work is done by the man). The power which has to be exerted by the motor remains the same since the reduced amount of work will be done in a correspondingly reduced period of time.