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:
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
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
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
A worker had done 150 J of work in 5 s. Calculate the power he generated.
Power (W) = 150 J = 30 W
5 s
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
PART 1
PART 2