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.
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.
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.
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 are forces that act between two objects that are not physically touching each other. Examples of non-contact forces include:
Opposite magnetic poles (N–S or S–N) attract each other:
Like magnetic poles (N–N or S–S) repel each other:
Opposite charges (+ and –) attract:
Like charges (– and –, or + and +) repel:
A gravitational force is experienced by any mass in a gravitational field.
Masses are attracted towards each other by gravitational force:
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.