Class 11 Physics Chapter 4: Laws of Motion – Notes & Questions

Class 11 Physics Chapter 4: Laws of Motion

Laws of Motion is one of the most important chapters in Class 11 Physics. It explains how forces affect the motion of objects and forms the foundation for solving many numerical problems in mechanics.

Chapter Focus: Newton's laws of motion, inertia, momentum, impulse, conservation of momentum, equilibrium, friction and centripetal force.

In this chapter:

• Force and Inertia
• Newton's First Law
• Momentum
• Newton's Second Law
• Impulse
• Newton's Third Law
• Conservation of Momentum
• Equilibrium of Forces
• Friction
• Centripetal Force

1. Force and Inertia

A force is an external interaction that can change the state of motion or shape of an object. Force is a vector quantity, so it has both magnitude and direction.

F = ma

Inertia

Inertia is the tendency of an object to resist any change in its state of rest or motion. The mass of an object is a measure of its inertia. Greater mass means greater inertia.

Type of Inertia	Example
Inertia of rest	A passenger moves backward when a stationary bus suddenly starts.
Inertia of motion	A passenger moves forward when a moving bus suddenly stops.
Inertia of direction	A passenger tends to move sideways when a bus takes a sharp turn.

2. 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 external unbalanced force acts on it.

This law is also called the law of inertia.

Key idea: If the net external force on a body is zero, its acceleration is zero.

ΣF = 0   ⇒   a = 0

3. Linear Momentum

Linear momentum is the product of the mass and velocity of an object. It is represented by p.

p = mv

Momentum is a vector quantity and its SI unit is kg m s^-1.

The direction of momentum is the same as the direction of velocity.

4. Newton's Second Law of Motion

Newton's second law relates the net external force acting on an object to the rate of change of its momentum.

F = dp/dt

For a body of constant mass:

F = ma

Thus, acceleration is directly proportional to the net force and inversely proportional to the mass of the body.

Remember: Always use the net force while applying Newton's second law.

5. Impulse

Impulse is the product of force and the time interval for which the force acts. It is equal to the change in momentum.

J = FΔt = Δp

Impulse is especially useful when a large force acts for a very short time, such as during a collision.

6. Newton's Third Law of Motion

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

F_AB = -F_BA

The action and reaction forces:

• are equal in magnitude,
• are opposite in direction,
• act on different bodies,
• occur simultaneously.

Examples include walking, swimming, recoil of a gun and rocket propulsion.

7. Law of Conservation of Linear Momentum

If the net external force acting on a system is zero, the total linear momentum of the system remains constant.

Total momentum before = Total momentum after

For two interacting bodies:

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂

8. Equilibrium of Forces

A body is said to be in equilibrium when the resultant of all forces acting on it is zero.

ΣF = 0

For equilibrium in two dimensions:

ΣF_x = 0    and    ΣF_y = 0

A body in equilibrium can either be at rest or move with constant velocity.

9. Friction

Friction is the force that opposes the relative motion or tendency of relative motion between two surfaces in contact.

Static Friction

Static friction acts when there is no actual relative motion between the surfaces. It adjusts its magnitude according to the applied force up to a maximum value.

f_s ≤ μ_sN

The maximum value of static friction is called limiting friction:

f_max = μ_sN

Kinetic Friction

Kinetic friction acts when two surfaces are actually sliding over each other.

f_k = μ_kN

Usually, kinetic friction is less than limiting static friction.

Rolling Friction

Rolling friction acts when an object rolls over a surface. It is generally much smaller than sliding friction, which is why wheels make movement easier.

10. Centripetal Force

When an object moves in a circular path, its velocity continuously changes direction. Therefore, it has an acceleration directed towards the centre of the circle. This is called centripetal acceleration.

a_c = v²/r

The force responsible for this acceleration is called centripetal force.

F_c = mv²/r

Using angular velocity:

F_c = mω²r

Important: Centripetal force is not a new type of force. It is the name given to the net force directed towards the centre of circular motion.

Examples of Centripetal Force

• Gravitational force keeps planets in orbit.
• Friction provides centripetal force for a vehicle on a level circular road.
• Tension can provide centripetal force when a stone is tied to a string.
• Normal reaction and friction together may provide the required force on a banked road.

Quick Revision

Concept	Important Formula
Momentum	p = mv
Newton's second law	F = dp/dt
Constant mass	F = ma
Impulse	J = Δp = FΔt
Limiting friction	fmax = μsN
Kinetic friction	fk = μkN
Centripetal acceleration	ac = v2/r
Centripetal force	Fc = mv2/r

Important Questions – Laws of Motion

Very Short Answer Questions

1. What is inertia?
2. What is the SI unit of force?
3. Define linear momentum.
4. State Newton's first law of motion.
5. What is impulse?
6. State Newton's third law of motion.
7. What is limiting friction?
8. Define coefficient of friction.
9. What is centripetal acceleration?
10. What is the condition for translational equilibrium?

Short Answer Questions

1. Explain the three types of inertia with suitable examples.
2. Why does a passenger move forward when a moving bus suddenly stops?
3. Why are action and reaction forces unable to cancel each other?
4. Explain the difference between static friction and kinetic friction.
5. Why is friction called a self-adjusting force in the case of static friction?
6. Explain the law of conservation of linear momentum.
7. Why does a cricketer move his hands backwards while catching a fast ball?
8. Why is it easier to pull a roller than to slide it?
9. Why does a vehicle require centripetal force while moving along a curved road?
10. Why is centripetal force always directed towards the centre?

Long Answer Questions

1. State and explain Newton's three laws of motion with everyday examples.
2. Derive the relation F = ma from Newton's second law of motion.
3. Explain the law of conservation of linear momentum and derive it for two interacting bodies.
4. Explain static friction, limiting friction and kinetic friction.
5. Derive the expression for centripetal force acting on a body moving in a circular path.

Important MCQs

1. The SI unit of momentum is:
   (a) N   (b) kg m/s   (c) J   (d) W
   Answer: (b) kg m/s
2. If the net force acting on a body is zero, its acceleration is:
   (a) maximum   (b) zero   (c) negative   (d) infinite
   Answer: (b) zero
3. Newton's second law gives the relation between force and:
   (a) displacement   (b) momentum   (c) energy   (d) power
   Answer: (b) momentum
4. The tendency of a body to resist a change in its state of motion is called:
   (a) momentum   (b) impulse   (c) inertia   (d) friction
   Answer: (c) inertia
5. Centripetal acceleration is directed:
   (a) away from the centre   (b) towards the centre   (c) along tangent   (d) vertically upward
   Answer: (b) towards the centre
6. The force of friction generally acts:
   (a) along the direction of motion
   (b) opposite to relative motion or its tendency
   (c) vertically upward
   (d) towards the centre
   Answer: (b)

Numerical Problems

Numerical 1: Force and Acceleration

A force of 20 N acts on a body of mass 5 kg. Find its acceleration.

a = F/m = 20/5 = 4 m/s²

Answer: 4 m/s²

Numerical 2: Momentum

Find the momentum of a body of mass 4 kg moving with a velocity of 6 m/s.

p = mv = 4 × 6 = 24 kg m/s

Answer: 24 kg m/s

Numerical 3: Impulse

A force of 50 N acts on an object for 0.2 s. Find the impulse produced.

J = FΔt = 50 × 0.2 = 10 N s

Answer: 10 N s

Numerical 4: Friction

A block experiences a normal reaction of 100 N. If the coefficient of kinetic friction is 0.2, calculate the kinetic friction.

f_k = μ_kN
= 0.2 × 100 = 20 N

Answer: 20 N

Numerical 5: Centripetal Force

A body of mass 2 kg moves in a circular path of radius 4 m with a speed of 8 m/s. Find the centripetal force.

F_c = mv²/r
= 2 × 8² / 4
= 32 N

Answer: 32 N

Assertion and Reason Questions

1. Assertion: A body continues in uniform motion when the net external force acting on it is zero.
   Reason: According to Newton's first law, a body resists a change in its state of motion.
   Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.
2. Assertion: Action and reaction forces do not cancel each other.
   Reason: They act on different bodies.
   Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.
3. Assertion: Friction can act even when an object is not moving.
   Reason: Static friction opposes the tendency of relative motion.
   Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.

Exam Tips for Laws of Motion

• Remember the exact statements of Newton's three laws.
• Practice free-body diagrams before solving force-based numericals.
• Always identify the net force rather than using a single force blindly.
• Learn the difference between static, limiting and kinetic friction.
• Practice momentum-conservation problems carefully with signs and directions.
• For circular motion, remember that centripetal force is directed towards the centre.

Class 11 Physics Chapter 4 FAQs

1. What is Laws of Motion in Class 11 Physics?

Laws of Motion is Chapter 4 of Class 11 Physics. It explains force, inertia, Newton's laws, momentum, impulse, friction, equilibrium and circular motion.

2. What is Newton's first law?

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

3. What is Newton's second law formula?

The general form is F = dp/dt. For constant mass, it becomes F = ma.

4. What is Newton's third law?

For every action, there is an equal and opposite reaction. The two forces act on different bodies.

5. What is momentum?

Momentum is the product of mass and velocity. Its formula is p = mv.

6. What is impulse?

Impulse is the product of force and the time interval for which the force acts. It is equal to the change in momentum.

7. What is friction?

Friction is the force that opposes relative motion or the tendency of relative motion between surfaces in contact.

8. What is the formula for kinetic friction?

The formula is f_k = μ_kN.

9. What is centripetal force?

Centripetal force is the net inward force required to keep an object moving along a circular path. Its magnitude is F_c = mv²/r.

10. Is centripetal force a separate type of force?

No. Centripetal force is not a separate fundamental force. It describes the net force directed towards the centre of circular motion.

Chapter 4 Formula Sheet

Topic	Formula
Momentum	p = mv
Newton's second law	F = dp/dt
Constant mass	F = ma
Impulse	J = Δp = FΔt
Equilibrium	ΣF = 0
Static friction	fs ≤ μsN
Limiting friction	fmax = μsN
Kinetic friction	fk = μkN
Centripetal acceleration	ac = v²/r
Centripetal force	Fc = mv²/r

One-Minute Revision

• Inertia: Resistance to change in motion.
• First law: Zero net force means zero acceleration.
• Momentum: p = mv.
• Second law: F = dp/dt; for constant mass, F = ma.
• Impulse: J = Δp.
• Third law: Action and reaction are equal and opposite.
• Conservation: Total momentum remains constant when external net force is zero.
• Friction: Opposes relative motion or its tendency.
• Centripetal force: Net inward force required for circular motion.

Conclusion

Class 11 Physics Chapter 4 – Laws of Motion is a foundation chapter for mechanics. A clear understanding of Newton's laws, free-body diagrams, friction, momentum and centripetal force will make many upcoming chapters easier. Regular practice of conceptual questions and numericals is the best way to master this chapter.

These notes are designed for quick revision, school examinations and CBSE preparation.