2.3 Newton's First Law

Newton's First Law: Inertia

An object's velocity stays constant (whether zero or nonzero) unless a net force acts on it. This tendency to resist changes in motion is called inertia. If the net force on an object is zero, the object is in equilibrium — it moves at constant velocity or remains at rest.

Net Force and Equilibrium

Net force is the vector sum of all forces acting on an object. When forces are balanced (net force = 0), an object experiences no acceleration — this includes objects at rest or moving with constant velocity in a straight line. Analyzing force diagrams (free-body diagrams) helps identify whether forces are balanced.

Mass and Inertia

Inertia depends on mass — more massive objects require larger forces to change their motion. Mass is a scalar quantity measuring an object's resistance to acceleration, distinct from weight (a force). Newton's First Law explains why passengers lurch forward when a car suddenly stops: their bodies resist the change in motion.

Key terms

Newton's First Law

An object's velocity remains constant unless acted upon by a net external force.

Inertia

The tendency of an object to resist changes in its motion; depends on mass.

Net force

The vector sum of all forces acting on an object.

Equilibrium

A state where net force equals zero, resulting in constant velocity (including rest).

Mass vs. Weight

Mass is a scalar measuring inertia; weight is the force of gravity on an object's mass.

Try an AP-style question

A car moves at constant velocity on a straight highway. What is the net force on it?

1. Zero
2. Forward, equal to its engine force
3. Upward
4. Backward, equal to friction