- 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, s1 = 8m and s2 = 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.s1 cosθ = 15 x 8cos 90° = 0J
When the coolie climb, thus θ = 90°
Work done, W2 = F-s2 cosθ = 15x 10 x cos0° = 150 J
Total work done by him W = W1 + W2 = 0 +150 = 150 J