Everything Science Grade 12

8.2 CHAPTER 8. WORK, ENERGY AND POWER

the displacement. Workis calculated as:

W = F· Δx cos θ. (8.1)

where F is the applied force, Δx is the displacement ofthe object and θ is the angle
between the applied force and the direction of motion.

F

θ Δx

F cos θ

Figure 8.1: The force F causes the object to bedisplaced by Δx at angle θ.

It is very important to note that for work to bedone there must be a component of
the applied force in thedirection of motion. Forces perpendicular to the direction of
motion do no work.

For example work is done on the object in Figure8.2,

F

Δx

(a)

F

Δ

y

(b)

Figure 8.2: (a) The force F causes the object to bedisplaced by Δx in the same direc-
tion as the force. θ = 0◦and cos θ = 1. Work is done in thissituation. (b) A force
F is applied to the object. The object is displaced by Δy at right angles to the force.
θ = 90◦and cos θ = 0. Work is not done in this situation.

Activity: Is work done?

Decide whether on notwork is done in the following situations. Remember

that for work to be donea force must be appliedin the direction of motion and

there must be a displacement. Give reasons for your answer.

1. Max pushes against awall and becomes tired.


2. A book falls off a table and free falls to the ground.