Interactions

Interactions occur between objects and are most easily represented by diagrams. No force occurs by itself, it is always as a result of a pull or push of one body on another. There are several examples:

Reaction force (contact)

An object at rest on a surface experiences reaction force from the table pushing up on the object. For example, a box on a table - there is the pull of the Earth down on the box and the push of the table up on the box.

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.

Friction (contact)

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

A box sliding down a ramp with acting forces

Air resistance (contact)

An object moving through the air experiences air resistance. For example, a skydiver falling through the air. There is contact between the body of the skydiver and the molecules that make up the air.

A box falling to Earth with the acting forces weight and air resistance

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. So if body "A" pushes on body "B", then body "B" pushes back equally on body "A".

Magnetic force (non-contact)

A magnetic force is experienced by any magnetic material in a magnetic field.

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 (non-contact)

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 (non-contact)

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

Masses are attracted towards each other by gravitational force:

The Earth and a satellite point to each other in space with arrows of equal size.

The Earth pulls on the satellite (weight) to keep it in orbit. However, the satellite also pulls back with an equal force on the Earth. Because the Earth is so large, this force has such a small effect on the Earth that it is not noticed.