2.1 Newton's First Law

Newton's First Law (Law of Inertia)

An object's velocity stays constant — including remaining at rest — unless a net external force acts on it. This tendency to resist changes in motion is called inertia. Motion doesn't require a force to continue; force is only needed to change motion (accelerate it).

Net Force and Equilibrium

The net force is the vector sum of all forces acting on an object. When net force equals zero, the object is in translational equilibrium — either at rest (static equilibrium) or moving at constant velocity (dynamic equilibrium). Zero net force does NOT mean zero forces are present, just that they cancel.

Inertial Reference Frames

Newton's First Law only holds true in an inertial reference frame — one that is not accelerating. In a non-inertial (accelerating) frame, objects appear to accelerate without any real force causing it, which would violate the law. Mass is the quantitative measure of an object's inertia.

Key terms

Newton's First Law

An object's velocity remains constant unless acted on by a net external force (Law of Inertia).

Inertia

The tendency of an object to resist changes in its state of motion.

Net force

The vector sum of all forces acting on an object.

Translational equilibrium

A state where the net force on an object is zero, so it's at rest or moving at constant velocity.

Inertial reference frame

A non-accelerating frame of reference in which Newton's First Law holds true.

Try an AP-style question

In which reference frame does Newton's first law hold?

1. Only frames at absolute rest
2. Any accelerating frame
3. Any rotating frame
4. An inertial (non-accelerating) frame