Newton’s Laws of Motion: The Foundation of Classical Physics

Newton’s Laws of Motion are basic principles of Physics. They explain how force affects an object's motion and how objects interact with one another.

To understand motion, it is not enough to know how fast an object is moving. We also need to know why its speed changes, when acceleration occurs, and what the net effect of the forces acting on it is.

Newton’s Laws of Motion explain this relationship through three important laws. In competitive exams, questions are asked based on concepts, formulas, and numerical problems.

The three laws are:

• Newton’s First Law – Law of Inertia
• Newton’s Second Law – Relation between Force and Momentum
• Newton’s Third Law – Action and Reaction

In NCERT Physics, these laws are explained along with momentum, impulse, conservation of momentum, equilibrium, and common forces in mechanics.

Newton’s First Law of Motion

Newton’s First Law says that an object at rest stays at rest, and an object in motion continues moving in a straight line with constant speed, unless an external unbalanced force acts on it.

This is also called the Law of Inertia. If no net external force acts on a body, its acceleration is zero. NCERT explains this law in the same way.

What is Inertia?

Inertia is the property of an object due to which it resists any change in its state of rest or motion.

Inertia depends on the mass of the object.

More mass → More inertia

So, heavier objects need more force to change their motion compared to lighter objects.

Types of Inertia

Type of Inertia	Meaning	Example
Inertia of Rest	Object at rest wants to stay at rest.	Passenger falls backward when a bus starts suddenly.
Inertia of Motion	Moving object wants to keep moving.	Passenger moves forward when a bus stops suddenly.
Inertia of Direction	Object resists change in direction.	Passenger leans when a vehicle takes a turn suddenly.

Daily Life Examples of First Law

• Passengers fall backward when a bus starts suddenly.
• Passengers move forward when a bus stops suddenly.
• Dust comes out when a carpet is shaken.
• Fruits fall when tree branches are shaken.
• Seat belts protect passengers during sudden braking.

Newton’s Second Law of Motion

Newton’s Second Law explains the relationship between force and acceleration. It also connects force with the rate of change of momentum.

This law is very important for numerical problems.

Formula of Second Law

F = dp/dt

Where:

• F = Force
• p = Momentum
• t = Time

If mass is constant:

F = ma

Where:

• m = mass
• a = acceleration

SI unit of force is Newton (N).

1 N = 1 kg m/s²

What is Momentum?

Momentum is the product of mass and velocity.

p = mv

Where:

• p = momentum
• m = mass
• v = velocity

SI unit of momentum is kg m/s.

Relationship Table

Quantity	Formula	SI Unit
Force	F = ma	Newton (N)
Momentum	p = mv	kg m/s
Acceleration	a = F/m	m/s²
Mass	m = F/a	kg
Velocity	v	m/s

Example of Second Law

If a 5 kg object has acceleration of 2 m/s²:

F = ma
F = 5 × 2
F = 10 N

So, the force is 10 N.

What is Impulse?

Impulse is the change in momentum when a force acts for a short time.

J = Δp

If force is constant:

J = FΔt

So,

Impulse = Change in Momentum

NCERT explains that a large force acting for a very short time is called an impulsive force.

Example of Impulse (Cricket)

A cricketer moves his hands backward while catching the ball. This increases time, reduces force, and makes the catch safer.

Newton’s Third Law of Motion

Newton’s Third Law says that every action has an equal and opposite reaction.

More clearly, when two objects interact, they apply equal and opposite forces on each other.

Action and Reaction Forces

Action and reaction forces always act on different objects.

Example:

Earth pulls an object downward, and the object also pulls Earth upward with equal force.

Examples of Third Law

Situation	Action	Reaction
Walking	Foot pushes ground backward	Ground pushes person forward
Swimming	Swimmer pushes water backward	Water pushes swimmer forward
Rocket	Gas is pushed downward	Rocket moves upward
Gun recoil	Bullet moves forward	Gun moves backward
Jumping	Person pushes ground downward	Ground pushes person upward

Important Points

• Forces are equal in magnitude
• Forces are opposite in direction
• They act at the same time
• They act on different objects
• They do not cancel each other

Balanced and Unbalanced Forces

Balanced forces have zero net force, while unbalanced forces produce motion or change in motion.

Type of Force	Net Force	Effect
Balanced Forces	0	No change in motion
Unbalanced Forces	Not 0	Motion or acceleration occurs

Examples

• Book on a table (balanced force)
• Kicking a football (unbalanced force)

Conservation of Momentum

If no external force acts on a system, total momentum remains constant.

Initial momentum = Final momentum

For two objects:

p₁ + p₂ = p₁′ + p₂′

Applications

• Gun recoil
• Collisions
• Rocket motion
• Explosions

Common Forces in Mechanics

Force	Meaning	Example
Gravitational Force	Attraction between masses	Object falling to Earth
Frictional Force	Opposes motion	Walking, braking
Normal Force	Support force from surface	Book on table
Tension Force	Force in rope or string	Hanging object
Applied Force	Direct push or pull	Pushing a box
Air Resistance	Opposes motion in air	Parachute falling slowly

What is Free Body Diagram (FBD)?

A Free Body Diagram shows all external forces acting on a body.

Steps:

• Identify the object
• Draw all forces acting on it
• Show correct directions
• Break forces into components if needed
• Apply Newton’s Second Law

Newton’s Laws and Equations of Motion

Equations of motion are not Newton’s laws. They are used for uniformly accelerated motion.

Equations of Motion (For Uniform Acceleration)

S.No	Relation Type	Formula
1	Velocity–Time	v = u + at
2	Displacement–Time	s = ut + 1/2 at²
3	Velocity–Displacement	v² = u² + 2as

Formula	Use
F = ma	Force and acceleration
p = mv	Momentum
v = u + at	Final velocity
s = ut + ½at²	Displacement
v² = u² + 2as	Motion without time

Important Exam Points

• First Law = Law of Inertia
• Second Law = F = ma or F = dp/dt
• Third Law = Action and Reaction
• Momentum = p = mv
• Force unit = Newton
• 1 N = 1 kg m/s²
• Impulse = Δp
• Conservation of momentum in isolated systems
• Action-reaction act on different bodies

Importance of Newton’s Laws

Newton’s Laws are the base of classical mechanics. They are used in walking, vehicles, sports, rockets, and machines.

For exams, understanding concepts like force direction, momentum, and action-reaction is more important than memorizing formulas.

Conclusion

Newton’s Laws of Motion explain the relationship between force and motion.

• First Law explains inertia
• Second Law explains force and acceleration
• Third Law explains action and reaction

For exams, focus on formulas like F = ma, p = mv, J = Δp, and understand real-life examples.

Most important point: Action and reaction forces never cancel each other because they act on different objects.

Important Links
Newton’s Laws of Motion	Gravitational Force
Rotational Motion	Physics