Contact forces

Contact forces are forces that act between two objects that are physically touching each other. Examples of contact forces include:

Normal contact force

An object at rest on a surface experiences a reaction force perpendicular (normal) to the surface. For example, a box on a table.

A box rests on a table. There are two arrows, equal in size but going in opposite directions, up and down, from the point where the box meets the table.

Tension

An object that is being stretched experiences a tension force. For example, a cable holding a ceiling lamp.

A box hangs from a rope. Two arrows which are equal in size act upwards and dowards from the top and bottom of the rope.

Friction

Two objects sliding past each other experience friction forces. For example, a box sliding down a slope.

A box rests on an incline. There are three arrows; one acting vertically downwards from the centre of the box’s base. One arrow acts perpendicular to the incline. One arrow acts up the incline.

Air resistance

An object moving through the air experiences air resistance. For example, a skydiver falling through the air.

A box falls from the sky. Two arrows, equal in size and opposite in direction act upwards from the box and downwards from the box

When a contact force acts between two objects, both objects experience the same size force, but in opposite directions. This is Newton's Third Law of Motion.