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[edit] What is a force?

Wikipedia has related information at Force

The simplest way to describe force is to say that it is a `push' or a `pull'. The push or pull on an object may cause either deformation or may change the state of motion of the object under consideration. The harder you 'push' or 'pull', the more force you are applying.

If we leave aside the deformation aspects, then force can be considered to produce change in the state of the motion of the object i.e. velocity. We have, though, experienced in real life that a 'push' or 'pull' does not always manifest in the change of motion. The reason is simple. A change in the state of motion requires a net force. For example, if the force is great enough to overcome friction the object being pushed or pulled will move. So long as the forces on an object are balanced (i.e. net force is zero), the state of motion described by "velocity" will remain same.

In fact, the acceleration of a body is directly proportional to the net force acting on it. The word net is important- forces are vectors and what matters in any situation is the vector sum of all the forces acting on an object.

The unit of force is the newton (symbol N). It is named after Sir Isaac Newton, whose three laws you will learn about shortly.

Force was first described by Archimedes. Archimedes of Syracuse (circa 287 BC - 212 BC), was a Greek mathematician, astronomer, philosopher, physicist and engineer. He was killed by a Roman soldier during the sack of the city, despite orders from the Roman general, Marcellus, that he was not to be harmed.