Forces and Motion

A force is that which can cause an object to change its velocity, i.e., to accelerate, or which can cause a flexible object to deform. 

This simulation illustrates the first point of this definition, namely that a force is capable of setting an object in motion, modifying its speed or changing its direction.

It is possible for a body to experience several forces, which can be added or subtracted. The net force acting on an object is the vectorial sum of all the forces acting on the object. If the forces are balanced, the net force is zero and there is no acceleration. If the forces are unbalanced, the net force is not zero and the object will accelerate in the direction of the net force. This is Newton's first law (1642-1727), also known as the law of inertia.

The tab with the 2 masses illustrates the influence of mass on motion, and more specifically on acceleration. Considering the same force F applied to 2 masses, the acceleration a is greater for the lighter body. This statement is Newton's second law:

a = F/m