Chapter 3 – Laws of Motion (Physics)
1. Introduction
Laws of Motion describe the relationship between force, mass, and motion.
They form the **foundation of classical mechanics** and are extensively used
in almost every chapter of Physics.
2. Force
A force is a push or pull that can:
- Change shape of an object
- Change speed
- Change direction of motion
- Start or stop motion
3. Inertia
Inertia is the tendency of a body to resist any change in its state of rest or motion.
| Type | Example |
|---|---|
| Inertia of rest | Passengers fall backward when bus starts |
| Inertia of motion | Passengers fall forward when bus stops |
| Inertia of direction | Stone flies tangentially from a sling |
4. Newton’s First Law of Motion
A body remains at rest or in uniform straight-line motion unless acted upon by an external force.
This law explains **inertia** and is also called the **law of inertia**.
5. Momentum
Momentum = mass × velocity
$$p = mv$$
Momentum is a vector quantity.
6. Newton’s Second Law of Motion
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.
$$F = \frac{dp}{dt}$$
For constant mass:
$$F = ma$$
7. Unit of Force
SI unit of force is Newton (N).
One Newton is the force required to accelerate a mass of 1 kg by $1 \, m/s^2$.
8. Newton’s Third Law of Motion
For every action, there is an equal and opposite reaction.
Action and reaction:
- Act on different bodies
- Are equal in magnitude
- Are opposite in direction
9. Conservation of Momentum
If no external force acts on a system, the total momentum of the system remains constant.
$$m_1u_1 + m_2u_2 = m_1v_1 + m_2v_2$$
10. Impulse
Impulse is the product of force and time.
$$Impulse = F \times t = \Delta p$$
Large force acting for short time produces significant impulse.
11. Applications of Newton’s Laws
- Motion of lift
- Motion on inclined plane
- Connected bodies
- Pulley problems
12. Free Body Diagram (FBD)
A Free Body Diagram shows all forces acting on a body.
Correct FBD is the **key to solving JEE problems**.
13. Friction
Friction is a force that opposes relative motion between two surfaces in contact.
Types of Friction
- Static friction
- Kinetic friction
- Rolling friction
14. Limiting Friction
$$f_{max} = \mu N$$
$\mu$ = coefficient of friction
$N$ = normal reaction
15. Common JEE Mistakes
- Wrong free body diagram
- Confusing action–reaction pair
- Ignoring direction of forces
- Assuming friction always equals $\mu N$
16. Important Formula Summary
| Concept | Formula |
|---|---|
| Momentum | $p = mv$ |
| Force | $F = ma$ |
| Impulse | $F t = \Delta p$ |
| Friction | $f = \mu N$ |
17. Final Revision Checklist
You have mastered this chapter if you can:
- Apply all three Newton’s laws correctly
- Draw accurate free body diagrams
- Solve friction and pulley problems
- Use conservation of momentum confidently