- A boy of mass 40 kg walks up a flight stairs to a vertical distance of 12 m, in a time interval of 40 s. At what rate is the boy doing work against force of gravity?
- A coolie is holding a bag by applying a force of 15 N. He moves forward and covers the horizontal distance of 8 m and then he climbs up and covers the vertical distance of 10 m. What will be the work done by him?

- Given, mass, m = 40 kg, time, t = 40 s

Energy = 30 kJ, Power = ?

Work done by the boy against force of gravity is equal to the gain in potential energy.

- Given, force, F = 15N, s
_{1} = 8m and s_{2} = 10m

As coolie is walking horizontally, therefore the angle between the bag and distance covered is 90°.

∴ Work done to cover the distance of 8 m, = F.s_{1} cosθ = 15 x 8cos 90° = 0J

When the coolie climb, thus θ = 90°

Work done, W_{2} = F-s2 cosθ = 15x 10 x cos0° = 150 J

Total work done by him W = W_{1} + W_{2} = 0 +150 = 150 J