Newton’s first law of motion

An object does not need a force to keep moving at a steady speed in a straight line. It needs a resultant force to change its motion.

Newton’s first law states that an object remains at rest or continues moving with constant velocity in a straight line unless a resultant external force acts on it. An external force is a force exerted on the object by something outside it. The resultant force is the combined effect of all the external forces acting on the object.

When the resultant force is zero, the object has no acceleration. Its velocity does not change: if it is stationary, it stays stationary; if it is moving, it continues at the same speed in the same direction. Zero resultant force does not necessarily mean that no forces act. Forces can act in opposite directions and balance each other.

A resultant force is needed to change an object’s speed or its direction of motion. For example, a sliding object may slow down because friction exerts a force on it, not because motion itself requires a force to continue.

The tendency of an object to resist a change in its motion is called inertia. An object at rest tends to remain at rest, while a moving object tends to maintain its velocity.