The graph shows the variation with time of the height through which a crane lifts a mass of
20 kg.
Assume the gravitational field strength g is 10 N / kg, and that the effects of air resistance and
friction are negligible.
What is the power output of the crane when the mass is at a height of 10 m?

Answer:

Power output in this question is about the change in potential energy (mgh) per unit time.
The mass and gravitational field strength are constant, so the change in potential energy with time is
directly related to the change in height with time.
In the graph above we can see that the change in height with time (the gradient) is constant from 15 s
to 35 s (which includes the time at which it is at the height of 10 m).So we can use this as the time
interval from which to calculate the power and energy change.
The change in height is 10 m (15 – 5 m) in this interval of 20 s. This relates to a change in potential
energy of 2000 J (mgh = 20×10×10) in the interval. The power output is given by change in potential
energy, divided by the time interval: 2000
20 = 100W.
The answer is D