This law is the result of one of those brilliant pieces of deduction, which from time to time produce an idea so simple that it is difficult to understand why it had eluded thinkers for so long. The law states that:

A body will continue in its state of rest, or of uniform motion in a straight line, unless an external force is applied to it.

 

 

This immediately explains the motion of the ball: once pushed and set it motion, it continues to move in a straight line until some other force intervenes.

 

Further deductions can be made from Newton’s first law.

-         If a body is at rest, or is moving with constant velocity, then there is no resultant force acting on it and any forces that so act must balance exactly, i.e. must be in equilibrium.

-         If the speed of a moving object is changing, there must be a resultant force acting on it.

-         If the direction of motion of a moving object is changing i.e. it is not moving in a straight line, there must be a resultant force acting on it. ( So there is always a force acting on a body acting on a body that is moving in a curve, even if the speed is constant.)

 

Newton’s first law in effect defines what force is i.e. force is quantity that, when acting on a body, changes the velocity of that body.