Last Updated: April 2026
Laws of Motion (NCERT Class 11, Chapter 5) is one of the most fundamental chapters in NEET Physics. It forms the basis for almost all of classical mechanics and consistently contributes 2–3 questions in NEET every year. More importantly, understanding Newton’s Laws is essential for solving problems in Work-Energy, Gravitation, Rotational Motion, and even Fluid Mechanics.
Chapter Overview
| Parameter | Details |
|---|---|
| NCERT Class | Class 11, Chapter 5 |
| NEET Questions (typically) | 2–3 per year |
| Difficulty | Moderate |
| Key Concepts | Newton’s 3 Laws, Friction, Momentum, Impulse, Circular Motion |
| Common Question Types | Numericals on F=ma, friction problems, pulley systems, circular motion |
Newton’s First Law of Motion — Law of Inertia
Statement: A body continues to be in a state of rest or uniform motion in a straight line unless acted upon by an external force.
- Inertia: The property of a body to resist change in its state of rest or motion
- Types of Inertia: Inertia of rest, Inertia of motion, Inertia of direction
- Inertia depends on: Mass (heavier objects have more inertia)
Examples for NEET:
- Passengers lean forward when brakes are applied → inertia of motion
- Dust falls off when a carpet is beaten → inertia of rest
- A ball rolled on a frictionless surface would roll forever → Newton’s 1st Law
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 force.
F = ma (Force = Mass × Acceleration)
In vector form: F⃗ = m × a⃗
| Quantity | Symbol | SI Unit | Dimensions |
|---|---|---|---|
| Force | F | Newton (N) = kg⋅m/s² | [MLT⁻²] |
| Mass | m | kilogram (kg) | [M] |
| Acceleration | a | m/s² | [LT⁻²] |
| Momentum | p = mv | kg⋅m/s | [MLT⁻¹] |
Impulse: J = F × t = Δp (Change in Momentum)
Unit of Impulse = Newton-second (N⋅s) = kg⋅m/s
Newton’s Third Law of Motion
Statement: For every action there is an equal and opposite reaction. Forces always occur in pairs.
- Action and reaction act on different bodies (not the same body)
- They are equal in magnitude and opposite in direction
- They act simultaneously
Examples:
- Rocket propulsion: exhaust gases pushed backward (action) → rocket moves forward (reaction)
- Walking: foot pushes ground backward (action) → ground pushes foot forward (reaction)
- Recoil of a gun: bullet pushed forward → gun pushed backward
Friction — Types and Formulae
| Type | Definition | Formula | Key Property |
|---|---|---|---|
| Static Friction | Friction before motion begins | fₛ ≤ μₛN | Variable (0 to μₛN); always ≥ kinetic friction |
| Kinetic Friction | Friction during sliding motion | fₖ = μₖN | Constant for given surfaces; less than max static |
| Rolling Friction | Friction during rolling | Very small | Least of all friction types |
Key Facts:
- μₛ > μₖ (static coefficient > kinetic coefficient) always
- Friction does NOT depend on area of contact
- Friction depends on: normal reaction (N) and nature of surfaces (μ)
- Normal Reaction (N) on horizontal surface = mg
- On inclined plane: N = mg cosθ
Important Formulae for NEET
- Net Force: F_net = ma
- Impulse-Momentum Theorem: J = Δp = mv − mu
- Friction force: f = μN
- Acceleration on incline (no friction): a = g sinθ
- Acceleration on incline (with friction): a = g(sinθ − μcosθ)
- Normal force on incline: N = mg cosθ
Solved NEET Problem
Problem: A block of mass 5 kg is placed on a horizontal surface. A force of 30 N is applied. If μₖ = 0.3 and g = 10 m/s², find the acceleration of the block.
Solution:
- Normal force N = mg = 5 × 10 = 50 N
- Kinetic friction fₖ = μₖN = 0.3 × 50 = 15 N
- Net force = 30 − 15 = 15 N
- Acceleration a = F_net/m = 15/5 = 3 m/s²
NEET MCQ Practice: Laws of Motion
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Frequently Asked Questions
How many questions come from Laws of Motion in NEET?
Laws of Motion typically contributes 2–3 questions in NEET Physics. The questions range from conceptual (which law explains X?) to numerical (calculate acceleration given F and m). Friction problems and pulley/wedge problems are common. Mastering this chapter also helps in Work-Energy, Circular Motion, and Gravitation.
What is the difference between static and kinetic friction?
Static friction acts when a body is at rest and prevents motion from starting — it is variable (0 to μₛN). Kinetic friction acts when a body is already sliding — it is constant (= μₖN). The coefficient of static friction (μₛ) is always greater than kinetic friction (μₖ) for the same surfaces.
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