Forces and interactions

Contact forces

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

Normal contact forces

An object at rest on a surface exerts a force on the surface. The reaction force acts at right angles to the surface and is known as the normal contact force. For example, a book 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.

Frictional forces

Objects moving across a surface exert force on the surface. The reaction force acts parallel to the surface and is known as a frictional force. 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 a frictional force between the object and the air. This frictional force is known as 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.

Non-contact forces

Non-contact forces are forces that act between two objects that are not physically touching each other. Examples of non-contact forces include:

Magnetic force

A magnetic force is experienced by any magnetic material in a magnetic field.The magnetic material does not need to be another magnet. For example, both iron and steel are magnetic materials.

Opposite magnetic poles (N–S or S–N) attract each other:

Two magnets are next to each other with North and South poles facing one another. Two equal arrows between the poles of the magnets point inwards towards each other.

Like magnetic poles (N–N or S–S) repel each other:

Two magnets are side by side with North poles facing each other. Two arrows between the poles of the magnets point outwards away from each other.

Electrostatic force

An electrostatic force is experienced by any charged particle in an electric field.

Opposite charges (+ and –) attract:

Large red circle contains ‘+’ symbol and smaller blue circle contains ‘-‘ symbol. Two arrows between the particles, point inwards towards each other.

Like charges (– and –, or + and +) repel:

Two blue circles contain ‘-‘ symbol. Two arrows between the particles point outwards away from each other.

Gravitational force

A gravitational force is experienced by any mass in a gravitational field.

Masses are attracted towards each other by gravitational force:

Graphic showing the moon and the earth. The moon has an arrow pointing at the earth, and the earth has an arrow pointing at the moon.

The Moon is attracted to the Earth, and the Earth is attracted to the Moon. Newton suggested that the Moon continually falls towards the Earth as it moves but the surface of the Earth continually drops away under the Moon. Hence, the Moon moves in a circular path around the Earth.