The addition of forces

Quite often more than one force acts on an object.

The sum of all the forces acting on a body is called the resultant force.

Imagine one person pulling a truck along – the truck will start to move. Now think what happens when two people pull. The truck will move faster because the force on the truck is the sum of the force applied by each person. (See Figures 1(a) and 1(b)).

If each person pulls with a force of 50 N the total force on the truck when two people pull it will be 100 N.

However when adding forces you must remember that forces are vectors and so you must allow for the direction in which the force is acting as well as its size. In Figure 2 the two men are pulling on ropes in different directions and this must be taken into account when working out the effective resultant force on the truck.

You should realise that the resultant force is bigger when both people pull in the same direction.

You can appreciate this by imagining one person pulling at right angles to the other. An example of two forces at right angles is shown in Figure 3 where the two tractors are pulling on the cow. The resultant force is going to pull the cow at an angle towards the bottom right of the picture as you look at it.

In Figure 2 the truck will move off along the rails because the rails also push on the truck.