7.1 | WORK, POWER AND ENERGY

work 

Work, W, is defined as the product of force, F, and displacement, s, in the direction of the force, that is 

W = Fs. 

S.I. unit for work - joule (J).

1 J = 1 Nm. 

Examples: 

1. Lifting an object vertically through a height of 1 m with a force of 30 N against the gravitational force. 

Force, F = 30 N Displacement = 1 m

W = Fs

W= 30 N x 1 m

W = 30 J 

2. Pushing a trolley over a distance of 10 m with a force of 20 N. 

Force, F = 20 N Displacement = 10 m

W = Fs

W= 20 N x 10 m

W = 200 J 

3. Pulling a drawer over a distance of 30 cm with a force of 3 N. 

Force, F = 3 N Displacement = 30 cm = 0.3 m

W = Fs

W= 3 N x 0.3 m

W = 0.9 J 

Energy and Power 

Energy - defined as the ability to do work. 

The S.I. unit for energy is joule (J).

 Power, P, is defined as the rate of doing work, W, that is: 

Power, P = Work done, W 

 Time taken, t

The S.I. unit for power is watt (W) 

1 W = 1 J s–1 

Example

1. A worker had done 150 J of work in 5 s. Calculate the power he generated. 

Power (W)  = 150 J = 30 W 

5 s

2. A boy pushed a 20 kg box over a distance of 2 m in 10 seconds. Calculate the power he generated. [g = 10 N kg–1] 

Work (J) = (20 × 10) N × 2 m 

= 200 N × 2 m 

= 400 J 

Power (W)  = 400 J/10s

= 40 W