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 anet external force. The net external force is the vector sum of all external forces.

We say that theforcesacting on a body arebalancedif 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 notaccelerating.

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