# Interactions

Interactions occur between objects and are most easily represented by diagrams. No 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 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.

### Friction (contact)

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

### Air resistance (contact)

An object moving through the air experiences . 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.

When a 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 material in a .

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

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

### Electrostatic force (non-contact)

An is experienced by any charged in an .

Opposite charges (+ and –) attract:

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

### Gravitational force (non-contact)

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

Masses are attracted towards each other by gravitational force:

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.