Forces are responsible for all the interactions between particles and objects. They can be put into two categories: contact forces and non-contact forces. Forces can be added up as vectors.

Two forces can be added together to find a resultant force. A single force can be resolved (broken down) into two component forces at right angles to each other.

In the diagram of a toy trailer below, when a child pulls on the handle, some of the 5 newton (N) force pulls the trailer upwards away from the ground and some of the force pulls it to the right.

Vector diagrams can be used to resolve the pulling force into a horizontal component acting to the right, and a vertical component acting upwards.

Draw a right-angled triangle to scale, in which each side represents a force. Try to choose a simple scale, for example 1 cm = 1 N. For the toy trailer example above, draw:

- a line representing the 5 N force at 37°
- a horizontal line ending directly below the end of the first line
- a vertical line between the ends of the two lines
- arrow heads to show the direction in which each force acts

Measure the lengths of the horizontal and vertical lines. Use the scale for the first line to convert these lengths to the corresponding forces.

The two component forces together have the same effect as the single force (in this example, the child’s pulling force).