C
CraftExam
high importancemedium8 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.

Practice This Concept

Learn the concept

Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

    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.

  2. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

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

Definition

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.

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.

Memory hook

Inertia means an object wants to continue its present state.

Examples and method

Worked example

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

Method to apply

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

Diagram support

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

How CBSE asks it

Exams ask everyday explanations: sudden bus stop, dust removed from carpet, or why seat belts are useful.

Avoid common mistakes

Common confusion

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

Common wrong answer

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

Exam tip

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

Quick check

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

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

Study the newton's first law (inertia) diagram carefully

Use the labelled diagram to keep newton's first law (inertia) clear in short answers and revision.

What this diagram makes clear

This diagram keeps the labels and direction of newton's first law (inertia) in the right order.

Where this helps in exams

Use this for labelled diagram work and short exam answers on newton's first law (inertia).

Revision cue

Revise newton's first law (inertia) through the labels before writing the answer.

Answer writing and exam use

1-mark answer

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.

2-mark answer

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. Law condition: motion changes only when an unbalanced external force acts. Mass is a measure of inertia. Passengers jerk forward when a moving bus suddenly stops because their bodies tend to continue moving forward due to inertia.

3-mark answer

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. Law condition: motion changes only when an unbalanced external force acts. Mass is a measure of inertia. A book on a table remains at rest because no unbalanced force changes its state. If pushed strongly enough, an unbalanced force makes it move. Exams ask everyday explanations: sudden bus stop, dust removed from carpet, or why seat belts are useful. Writing 'passenger is pushed forward by a forward force' is usually wrong; the forward motion continues due to inertia.
MCQ Quiz

Practice this concept with focused MCQs

Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.

10 MCQs5 MinutesInstant Results
Practice This Concept

Help improve this page

Found something confusing, incorrect, or missing?

Practice This Concept