CBSE Class 11 Physics Revision Notes Chapter 4 Laws of Motion

Laws of Motion explain how external forces change the state of rest or motion of a body. For CBSE Class 11 Physics, the chapter connects force with inertia, momentum, acceleration, equilibrium, friction and circular motion.

Motion can be described using displacement, velocity and acceleration. However, these quantities do not explain what causes motion to change. Laws of Motion introduce force as the external influence responsible for acceleration.

These CBSE Class 11 Physics Revision Notes Chapter 4 follow the 2026–27 chapter sequence. They cover inertia, Newton’s laws, momentum, impulse, equilibrium, friction and circular motion through compact explanations and formulas.

Key Takeaways

• Zero net force: A body remains at rest or moves with uniform velocity when the net external force is zero.
• Newton’s second law: Net force equals the rate of change of momentum, F = dp/dt.
• Action-reaction pair: The two forces are equal and opposite but act on different bodies.
• Centripetal force: A body of mass m moving at speed v in a circle of radius R requires mv²/R towards the centre.

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Access Class 11 Physics Chapter 4 Laws of Motion Notes in 30 Minutes

These Class 11 Physics Laws of Motion Notes follow the progression from inertia to force-based problem-solving.

From Aristotle’s Fallacy to the Law of Inertia

The modern laws of motion developed after scientists questioned the belief that continuous force was necessary for continuous motion.

Aristotle’s View of Motion

Aristotle believed that an external force was required to keep a body moving. Everyday observations appeared to support this idea because moving objects usually slow down and stop.

The flaw lies in ignoring friction. A toy car stops because friction acts opposite to its motion. Without friction, no external force would be required to maintain uniform motion.

Galileo’s Inclined Plane Reasoning

Galileo studied the motion of objects on inclined planes.

• An object moving down an inclined plane accelerates.
• An object moving up an inclined plane slows down.
• A body on a frictionless horizontal plane should move with constant velocity.

In the double inclined plane experiment, a ball rolling down one plane climbs the other to nearly its original height. Reducing the second slope increases the distance travelled.

If the second plane becomes horizontal and friction is absent, the ball continues moving indefinitely.

Inertia and Its Meaning

Inertia is the resistance of a body to a change in its state of rest or uniform linear motion.

If the net external force is zero:

• A stationary body remains stationary.
• A moving body continues with constant velocity.
• The acceleration of the body remains zero.

Rest and uniform linear motion are therefore equivalent states from the viewpoint of force.

Newton’s Laws of Motion in Class 11 Physics Chapter 4

Newton developed three laws that connect force with changes in motion. Together, they form the foundation of classical mechanics.

Newton’s First Law of Motion

Every body continues in its state of rest or uniform motion in a straight line unless an external force changes that state.

The law may be written as:

Net external force = 0
Therefore, acceleration = 0

Newton’s first law gives a precise statement of the law of inertia.

A book resting on a table remains at rest because its weight and the normal reaction balance each other. The body is unaccelerated because the net force is zero.

A passenger falls backwards when a stationary bus starts suddenly. The feet move with the bus due to friction, while the upper body tends to remain at rest.

Momentum

Linear momentum is the product of the mass and velocity of a body.

p = mv

Here:

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

Momentum is a vector quantity. Its direction is the same as the direction of velocity.

Its SI unit is kg m s⁻¹.

A heavy vehicle has more momentum than a light vehicle moving at the same speed. A faster body also has more momentum than the same body moving slowly.

Newton’s Second Law of Motion

The rate of change of momentum of a body is directly proportional to the applied force. The momentum changes in the direction of the force.

F = dp/dt

For a body of constant mass:

p = mv

Therefore:

F = m(dv/dt)

F = ma

Here:

• F = net external force
• m = mass
• a = acceleration

The SI unit of force is newton.

1 N = 1 kg m s⁻²

One newton produces an acceleration of 1 m s⁻² in a body of mass 1 kg.

Important Points About Newton’s Second Law

• It is a vector law.
• The force means the net external force on the body.
• Internal forces are not included when considering an entire system.
• Force at a particular instant determines acceleration at that instant.
• Force and acceleration have the same direction.
• Force need not have the same direction as velocity.

The component form is:

Fx = max
Fy = may
Fz = maz

Impulse and Change in Momentum

A large force may act for a very short interval and produce a measurable change in momentum. Such a force is called an impulsive force.

Impulse = Force × time interval

I = FΔt

According to Newton’s second law:

I = Δp

Therefore:

FΔt = pf − pi

A cricketer draws the hands backwards while catching a fast ball. This increases the stopping time and reduces the average force on the hands.

Newton’s Third Law of Motion

To every action, there is an equal and opposite reaction.

For two interacting bodies A and B:

FAB = −FBA

This means the force on A due to B equals the negative of the force on B due to A.

Features of Action-Reaction Forces

• They are equal in magnitude.
• They act in opposite directions.
• They act at the same time.
• They act on different bodies.
• They arise from the same interaction.
• They cannot cancel when studying one body.

When a person pushes a wall, the wall pushes the person with an equal force in the opposite direction.

While walking, the foot pushes the ground backwards. The ground applies a forward frictional force on the person.

Conservation of Linear Momentum

Newton’s second and third laws lead to the law of conservation of momentum.

The total momentum of an isolated system remains constant when no net external force acts on it.

Momentum Conservation in an Isolated System

Consider two interacting bodies A and B.

From Newton’s third law:

FAB = −FBA

If the forces act for the same time interval Δt:

FABΔt = −FBAΔt

Since impulse equals change in momentum:

ΔpA = −ΔpB

Therefore:

ΔpA + ΔpB = 0

The total momentum remains unchanged.

Collision Between Two Bodies

Suppose two bodies have initial momenta pA and pB. After collision, their momenta become p′A and p′B.

The conservation equation is:

pA + pB = p′A + p′B

For motion along a straight line:

m1u1 + m2u2 = m1v1 + m2v2

This equation applies when the net external force on the system is zero.

The total momentum remains conserved in both elastic and inelastic collisions. Kinetic energy conservation requires a separate condition.

Equilibrium of a Particle and Free-Body Diagrams

A particle is in equilibrium when the vector sum of all external forces acting on it is zero.

A particle in equilibrium may be:

• At rest
• Moving with uniform velocity

Conditions for Equilibrium

For two forces:

F1 + F2 = 0

Therefore:

F1 = −F2

The two forces must be equal, opposite and collinear.

For three concurrent forces:

F1 + F2 + F3 = 0

The three forces can be represented by the sides of a closed triangle taken in order.

For several forces:

ΣF = 0

Resolving Forces into Components

Equilibrium must hold separately along each coordinate axis.

ΣFx = 0
ΣFy = 0
ΣFz = 0

A suitable coordinate system can simplify the calculation. For an inclined plane, one axis is usually taken parallel to the plane.

Steps for Drawing a Free-Body Diagram

A free-body diagram shows one chosen body and every external force acting on it.

1. Draw the complete physical arrangement.
2. Select one body or group of bodies as the system.
3. Isolate that system from its surroundings.
4. Draw every external force acting on the selected system.
5. Do not include forces exerted by the system on its surroundings.
6. Mark known directions such as weight, normal reaction and tension.
7. Choose convenient coordinate axes.
8. Resolve inclined forces into components.
9. Apply ΣF = ma along each axis.

Internal forces cancel only when the connected bodies are treated as one complete system.

Common Forces in Mechanics

Mechanics problems generally involve weight, normal reaction, tension and friction. Each force has a specific direction and physical origin.

Gravitational Force or Weight

The Earth attracts every body towards its centre.

Weight is given by:

W = mg

Here:

• m = mass of the body
• g = acceleration due to gravity

Weight acts vertically downwards.

Normal Reaction

When two surfaces touch, each surface exerts a contact force on the other. The component perpendicular to the surface is the normal reaction.

The normal reaction is represented by N or R.

It is not always equal to mg. Its magnitude depends on the other forces and the acceleration perpendicular to the surface.

For a body resting on a horizontal surface with no other vertical force:

N = mg

Tension in a String

Tension is the force transmitted through a stretched string, rope or cable.

For an ideal string:

• The string is massless.
• The string is inextensible.
• Tension acts along the string.
• A smooth pulley changes the direction of tension.
• The tension remains the same throughout the string.

Tension always pulls a body. A string cannot push an attached body.

Frictional Force

Friction is the tangential component of contact force between two surfaces.

It opposes actual relative motion or the tendency of relative motion.

Friction acts along the common surface of contact.

Friction in Laws of Motion

Friction develops because the surfaces in contact interact at microscopic points. Its direction must be decided from the relative motion or impending motion.

Static and Limiting Friction

Static friction acts when there is no relative sliding between the surfaces.

It is a self-adjusting force:

0 ≤ fs ≤ fs,max

The maximum value of static friction is called limiting friction.

fs,max = μsN

Here:

• μs = coefficient of static friction
• N = normal reaction

Static friction does not always equal μsN. This equality applies only when the body is about to slide.

Kinetic Friction

Kinetic friction acts when one surface slides over another.

fk = μkN

Here:

• μk = coefficient of kinetic friction
• N = normal reaction

Usually:

μk < μs

Therefore:

fk < fs,max

Less force is generally required to keep a body sliding than to start its motion.

Rolling Friction

Rolling resistance acts when a body rolls over a surface.

It is much smaller than sliding friction. This is why wheels and ball bearings make it easier to move heavy loads.

For the same surfaces:

Rolling friction < Kinetic friction < Limiting friction

Why Friction Is Necessary

Friction may produce wear and heat, but many everyday actions depend on it.

• Walking requires static friction between the feet and the ground.
• A car accelerates because the road exerts friction on its tyres.
• Brakes use friction to reduce motion.
• Writing requires friction between the writing instrument and the surface.
• A vehicle turns because friction can provide centripetal force.

Lubricants, ball bearings and air cushions reduce friction where energy loss must be controlled.

Circular Motion and Centripetal Force

A body moving with uniform speed in a circle still accelerates because its velocity changes direction continuously.

The acceleration points towards the centre of the circular path.

ac = v²/R

Centripetal Force

The inward net force producing centripetal acceleration is called centripetal force.

Fc = mv²/R

Here:

• m = mass
• v = speed
• R = radius of the circular path

Centripetal force is not a separate type of force. It is the name given to the net inward force.

It may be provided by:

• Tension for a stone tied to a string
• Gravitational force for a planet
• Friction for a car taking a turn
• Normal reaction on a banked road

Motion of a Car on a Level Road

A car moving on a level circular road experiences:

• Weight mg downward
• Normal reaction N upward
• Friction towards the centre

Vertical equilibrium gives:

N = mg

Static friction provides the required centripetal force:

mv²/R ≤ μsN

Using N = mg:

v² ≤ μsRg

Therefore, the maximum safe speed is:

vmax = √(μsRg)

The maximum safe speed increases with the radius and coefficient of static friction.

Motion of a Car on a Banked Road

On a banked road, the outer edge is raised above the inner edge. A horizontal component of the normal reaction acts towards the centre.

At the optimum speed, friction is unnecessary.

Vertical balance:

N cos θ = mg

Horizontal component:

N sin θ = mv²/R

Dividing the equations:

tan θ = v²/Rg

Therefore:

v = √(Rg tan θ)

Banking reduces dependence on friction and allows safer turning at higher speeds.

Laws of Motion Formula Sheet

Important Distinctions in Laws of Motion

Useful Links for Class 11 Physics