(a) Define power and state its SI unit.
(b) A gardener pushes a lawn-mower along a horizontal lawn with a constant horizontal force of 60N so that it moves at a steady speed of 1.5ms−1. Without doing any numerical calculation, explain how the power developed by the gardener will change if the speed is doubled while the force remains the same. Give a reason for your answer based on the relationship between the relevant physical quantities.