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.

Resultant force and motion

The resultant force is the overall force on an object after all the forces acting on it have been combined, taking their directions into account. It determines whether the object’s motion changes.

Zero resultant force

When the resultant force is zero, the object’s velocity does not change:

• An object at rest remains at rest.
• A moving object continues in a straight line at constant speed.

A zero resultant force does not necessarily mean that no forces act on the object. Forces may act but balance one another, giving an overall force of zero.

Non-zero resultant force

When the resultant force is non-zero, the object’s velocity changes. It may speed up, slow down, change direction, or experience a change in both speed and direction.

Velocity includes direction as well as speed. For example, an object moving around a bend changes direction, so its velocity changes even if its speed stays constant.