The Online Library of Knowledge

# Motion

Motion is any kind of movement. Motion is caused by forces. A still or stationary object does not move unless a force acts on it to start it going. Once it is moving it carries on at the same speed in a straight line unless a force makes it speed up, change direction or slow down and stop. There are many kinds of movement. An object going in a straight line like a rocket shooting through deep space has linear motion. An object going around a central point like a ball swung round on a string has circular motion. An object moving to and fro like a pendulum or a playground swing has reciprocating motion. An object that twists around like a wheel or a screwdriver has rotary motion.

## Laws of motion

The English scientist Sir Isaac Newton (1642–1727) proposed three laws of motion in 1687. The first law states an object at rest will remain at rest unless acted on by a force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by a force. This law is also known as the law of inertia.

The second law states that the greater the mass of an object, the greater the amount of force needed to move it. The third law states that for every force, there is a reaction force that is equal in size, but one that acts in an opposite direction.