medium difficulty4 min

Newton's First Law (Inertia)

Newton's first law states that an object remains at rest or in uniform straight-line motion unless acted upon by an unbalanced external force.

In one line: Inertia means an object wants to continue its present state.

Practice This Concept

Understand it

This law explains inertia, the tendency of a body to resist change in its state of rest or motion. Greater mass means greater inertia, so heavier objects resist change more.

Example

Passengers jerk forward when a moving bus suddenly stops because their bodies tend to continue moving forward due to inertia.

Rule to remember

Law condition: motion changes only when an unbalanced external force acts.

Mass is a measure of inertia.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1A book on a table remains at rest because no unbalanced force changes its state.
2If pushed strongly enough, an unbalanced force makes it move.

Method you can reuse

State initial state, identify sudden change, mention inertia, then explain how the body resists that change.

Visual tip

A bus-stop sketch with passenger direction arrows can help explain inertia of motion.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why does a passenger move forward when a bus stops suddenly?

Think first, then show answer

The passenger's lower body stops with the bus, but the upper body tends to keep moving forward due to inertia of motion.

Common wrong answer

Writing 'passenger is pushed forward by a forward force' is usually wrong; the forward motion continues due to inertia.

Don't mix this up

Students often say inertia is a force, but inertia is a property of matter, not a force.

Exam tip

In seat-belt questions, mention inertia of motion and sudden stopping of the vehicle.