Class 9 & 10 Physics

What Are Newton's Laws of Motion?

Newton's three laws of motion help us understand why objects remain at rest, start moving, speed up, slow down or change direction. In this article, we explain each law in simple language with everyday examples, formulas and solved numerical problems.

In simple words: Newton's laws explain how forces affect the motion of objects.

1. What Are Newton's Laws of Motion?

Sir Isaac Newton gave three fundamental laws that describe the relationship between force and the motion of an object. These laws form an important foundation of classical mechanics.

The three laws are commonly called the law of inertia, the law of acceleration, and the law of action and reaction.

Think about a bus: When a stationary bus starts suddenly, passengers may lean backwards. When the moving bus stops suddenly, passengers may move forwards. These everyday observations are explained by Newton's laws.

2. Force and Motion: The Basic Idea

A force is a push or pull that can change the state of motion of an object. It can make a stationary object move, stop a moving object, change its speed or change its direction.

Quantity	Meaning	SI Unit
Force	A push or pull that can change motion.	newton (N)
Mass	Measure of the amount of matter in an object and its inertia.	kilogram (kg)
Acceleration	Rate at which velocity changes with time.	m/s²
Momentum	Product of mass and velocity.	kg m/s

Important: A force does not always mean that an object must move. If forces are balanced, the object's state of motion does not change.

3. Newton's First Law of Motion

Newton's first law states that an object remains at rest or continues to move with uniform velocity in a straight line unless an unbalanced external force acts on it.

Simple meaning: Objects do not change their state of motion on their own. They need an unbalanced force to do so.

Inertia

The tendency of an object to resist any change in its state of rest or motion is called inertia. Greater the mass of an object, greater is its inertia.

Type of Inertia	Example
Inertia of rest	Dust comes out of a carpet when the carpet is beaten.
Inertia of motion	A passenger moves forward when a moving bus stops suddenly.
Inertia of direction	A passenger leans sideways when a moving car takes a sharp turn.

Another example: When a coin is placed on a card over a glass and the card is flicked quickly, the coin tends to remain at rest and falls into the glass.

4. Newton's Second Law of Motion

Newton's second law connects force, mass and acceleration. It states that the rate of change of momentum of an object is proportional to the applied force and takes place in the direction of the force.

Momentum

Momentum is the product of mass and velocity.

p = mv

For a body of constant mass, Newton's second law gives the familiar relation:

F = ma

Here:

F = net force in newtons
m = mass in kilograms
a = acceleration in metres per second squared

Key idea: For the same mass, a greater net force produces greater acceleration. For the same force, a larger mass produces smaller acceleration.

5. Newton's Third Law of Motion

Newton's third law states that for every action, there is an equal and opposite reaction.

The action and reaction forces are equal in magnitude and opposite in direction. Importantly, they act on different objects, so they do not simply cancel each other.

Situation	Action	Reaction
Walking	Your foot pushes the ground backwards.	The ground pushes your foot forwards.
Swimming	A swimmer pushes water backwards.	Water pushes the swimmer forwards.
Rocket launch	Rocket gases are pushed downwards.	The gases exert an upward force on the rocket.
Jumping	You push the ground downwards.	The ground pushes you upwards.

Remember: Action and reaction forces occur simultaneously and act on two different objects.

6. Newton's Three Laws at a Glance

Law	Main Idea	Common Example
First law	An object resists a change in its state of motion.	Passenger moves forward when a bus stops suddenly.
Second law	Net force produces acceleration; for constant mass, F = ma.	A football accelerates more when kicked harder.
Third law	Forces occur in equal and opposite action-reaction pairs.	Walking, swimming or rocket propulsion.

7. Solved Numerical Examples

Numerical 1: Finding Force

A force acts on an object of mass 5 kg and produces an acceleration of 4 m/s². Find the force.

Given: m = 5 kg and a = 4 m/s²

Formula:

F = ma

F = 5\times4

F = 20\,\mathrm{N}

Answer: The force acting on the object is 20 N.

Numerical 2: Finding Acceleration

A net force of 30 N acts on a body of mass 6 kg. Find its acceleration.

Given: F = 30 N and m = 6 kg

From :

a = \frac{F}{m}

a = \frac{30}{6}

a = 5\,\mathrm{m/s^2}

Answer: The acceleration of the body is 5 m/s².

Numerical 3: Finding Mass

A force of 24 N produces an acceleration of 3 m/s² in an object. Find its mass.

Given: F = 24 N and a = 3 m/s²

m = \frac{F}{a}

m = \frac{24}{3}

m = 8\,\mathrm{kg}

Answer: The mass of the object is 8 kg.

Numerical 4: Comparing Acceleration

The same force of 20 N acts separately on two objects of masses 5 kg and 10 kg. Find their accelerations.

For the 5 kg object:

a_1 = \frac{20}{5} = 4\,\mathrm{m/s^2}

For the 10 kg object:

a_2 = \frac{20}{10} = 2\,\mathrm{m/s^2}

Answer: The lighter object has greater acceleration under the same force: 4 m/s² compared with 2 m/s².

8. Common Mistakes Students Make

Thinking that an object must have a force acting on it to continue moving at constant velocity.
Confusing balanced forces with the absence of all forces.
Using F = ma without identifying the net force when several forces act on an object.
Forgetting that mass is measured in kilograms in SI calculations.
Thinking that action and reaction forces act on the same object.
Writing the SI unit of force as kg or m/s instead of newton (N).

9. Quick Revision

First law: An object resists changes in its state of motion. This is the law of inertia.

Second law: For constant mass, .

Momentum: .

Third law: Every action has an equal and opposite reaction.

Mass and inertia: Greater mass means greater resistance to a change in motion.