Lesson 1: Work

Work is done when a force acts on a moving body. Work is done whenever a force moves something.

• Work is done whenever a force is used to make something move. 
• Work is done when there is movement against an opposing force.
• Doing work requires energy or is a transfer of energy
• As you do this work, energy is transferred. 
• Work done = Energy Transferred

Everyday examples of work include walking upstairs, lifting heavy objects, playing tug of war and pushing a shopping trolley.

Whenever work is done, energy is transferred from one place to another. The total amount of energy remains constant.

Figure 1: A man pushing a box up a ramp.

The S.I unit of the work is joule.

1 joule work is defined as the amount of work done to cover a distance of 1 m in the direction of force when the force of 1N is applied on the body.

 Work is done against two forces

 1. Work against the gravity.

 2. Work against friction.

Here is the equation that relates to work done, force applied, and distance moved in the direction of the force.

where:

W is measured in joules, J

F is measured in newtons, N

d is measured in metres, m

Figure 2: In the example above, 10 N is applied to move the box 2 m.  In this example the work done is against friction. 

Work done = 10 × 2 = 20 J

Work done has the same units as energy - joules. This is because energy is the ability to do work. You must have energy to do work. In the example above, a person could not push the suitcase (and so do work) without energy. Work done is equal to energy transferred, ie work = energy transferred.

Figure 3:  A weightlifter lifts a barbell weighing 50 kg and displaces it from the ground by 2 m. Here, the work done upon the barbell is against gravity

W= F x D

W= 50 x 2 = 100 J

A man lifts a box with a mass of 5 kg from the floor to a shelf 2 m high.  How much work is done?

Figure 4: A man lifting a box

The opposing force in this example is the weight of the brick.  On earth a mass of 1 kg is equal to 10 newtons (10 N).  So, the weight of the brick is 50 N.  

Work done (J) = force (N) x distance moved (m)

50 N x 2 m = 100 j