Newton’s First Law of Motion

Newton's first law of motion states that a body at rest remains at rest and a body moving in a straight line at a constant velocity remains moving in a straight line at a constant velocity unless acted on by a net external force. The net external force is the vector sum of all external forces.

We say that the forces acting on a body are balanced if the net force is zero. If the net force acting on a body is zero that object is said to be in equilibrium. By Newton's first law, an object in equilibrium is either at rest or moving with constant velocity; that is, it is not accelerating.

Newton's First Law of Motion is often called the Principle of Inertia.