Lesson–4 : Laws of Motion

Stage–1 : Detailed Intermediate Notes

---

🔹 1. Introduction

The Laws of Motion form the foundation of classical mechanics. They explain the relationship between force, mass, and motion of bodies.

This chapter is extremely important for:

• Intermediate Board Examinations
• IIT JEE Main and Advanced
• Understanding advanced topics like circular motion and work-energy

---

🔹 2. Force

A force is an external agent which can:

• Change the state of rest of a body
• Change the state of uniform motion
• Change the direction of motion
• Change the shape or size of a body

SI unit: Newton (N)

1 Newton is defined as the force which produces an acceleration of 1 m/s² in a body of mass 1 kg.

Force is a vector quantity because it has both magnitude and direction.

---

🔹 3. Types of Forces

Contact Forces

• Frictional force
• Normal reaction
• Tension
• Applied force

Non-Contact Forces

• Gravitational force
• Magnetic force
• Electrostatic force

In most problems of this chapter, we deal mainly with gravitational force and contact forces.

---

🔹 4. Inertia

Inertia is the natural tendency of a body to resist any change in its state of rest, motion, or direction.

Mass is a measure of inertia. Greater the mass of a body, greater is its inertia.

Types of Inertia

(a) Inertia of Rest

The tendency of a body at rest to remain at rest.

• Dust particles fall when a carpet is beaten
• Passenger falls backward when a bus starts suddenly

(b) Inertia of Motion

The tendency of a moving body to continue in motion.

• Passenger falls forward when a moving bus stops suddenly
• Luggage keeps moving when brakes are applied

(c) Inertia of Direction

The tendency of a body to resist change in direction.

• A stone released from circular motion moves tangentially
• Mud flies off from rotating tyres

---

🔹 5. Newton’s First Law of Motion

Statement:
A body continues in its state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.

This law is also known as the Law of Inertia.

Explanation

• If no external force acts, velocity remains constant
• Force is required only to change the state of motion

This law explains why passengers experience jerks in buses.

---

🔹 6. Momentum

Momentum is defined as the quantity of motion possessed by a body.

Mathematical expression:

p = mv

Where:

• m = mass of the body
• v = velocity of the body

Momentum is a vector quantity.

SI unit: kg m s⁻¹

A heavy truck moving slowly can have greater momentum than a light object moving fast.

---

🔹 7. Newton’s Second Law of Motion

Statement:
The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force.

Mathematical Form

F = ma

This law gives the quantitative definition of force.

It is the most important law for solving numerical problems.

---

🔹 8. Impulse

Impulse is defined as the product of force and the time for which the force acts.

Impulse = F × t = Change in momentum

Examples

• Catching a cricket ball by pulling hands backward
• Airbags in cars
• Boxing gloves

Longer contact time reduces force and prevents injury.

---

🔹 9. Newton’s Third Law of Motion

Statement:
For every action, there is an equal and opposite reaction.

Important Points

• Action and reaction act on different bodies
• They are equal in magnitude and opposite in direction
• They do not cancel each other

Examples

• Walking
• Swimming
• Rocket propulsion
• Recoil of a gun

---

🔹 10. Summary

• Force causes change in motion
• Mass measures inertia
• Momentum links mass and velocity
• F = ma is the most used equation
• Impulse equals change in momentum
• Action and reaction act on different bodies

---

Stage–1 Completed ✔
Next: Stage–2 (Intermediate Important Questions & Numericals)