Laws of Motion: Newton's Three Laws Made Simple

Q: A book lies at rest on a table. Which of these is a correct action–reaction pair under Newton's third law?

Action–reaction pairs are always equal, opposite, and act on two different bodies. The book pushes down on the table (action) and the table pushes up on the book (reaction) — different bodies, so this is the true pair. Option A is a trap: the book's weight and the normal force are equal and opposite but both act on the SAME body (the book), so they are balanced forces, not a third-law pair.

A from-scratch guide to inertia, momentum, force and friction — Newton's three laws explained with everyday Indian examples, a fully solved numerical, and free-body diagrams, built for CBSE Class 11 and your JEE/NEET/CUET foundation.

By the PadhoDost Team · 📖 8 min read · Updated 4 August 2026

Part of Class 11 (CBSE) prep

🧠 The Bus That Throws You Backward

Ever noticed how, the moment a stationary bus suddenly moves, you get pushed backward — and when it brakes hard, you lurch forward? No ghost is pushing you. Your body simply wants to keep doing what it was already doing. That stubborn tendency is called inertia, and it is the doorway to this entire chapter. Padho, dost — once you feel this in your bones, Newton's laws stop being formulas and start being plain common sense.

Newton's First Law: The Law of Inertia

A force is simply a push or a pull. Newton's First Law says: every object stays at rest, or keeps moving with uniform velocity in a straight line, unless a net external force acts on it. Objects never change their velocity on their own — they always need a reason. This 'laziness' to change motion is inertia, and heavier objects have more of it: pushing a loaded thela is far harder than pushing an empty one, because more mass means more inertia. Inertia shows up in three ways — inertia of rest (a coin stays put when you flick the card under it), inertia of motion (you jerk forward when a bus brakes), and inertia of direction (you are flung outward on a sharp turn).

First Law: if net force ΣF = 0, then acceleration a = 0, so velocity stays constant. Inertia is measured by mass — more mass, more inertia.

Newton's Second Law: Force = Mass × Acceleration

The first law tells us force changes motion; the second law tells us by how much. First meet momentum: p = mv, the 'quantity of motion' a body carries (units: kg·m/s). A loaded truck at 10 km/h is much harder to stop than a cycle at 10 km/h, because the truck has far more momentum. Newton's Second Law says the net force equals the rate of change of momentum. When the mass stays constant, this boils down to the famous F = ma. Remember: net force and acceleration always point in the same direction.

Momentum: p = mv. Second Law: F = Δp/Δt = ma (for constant mass). SI unit of force is the newton (N): 1 N = 1 kg·m/s², the force that gives a 1 kg mass an acceleration of 1 m/s².

📝 Worked Example: Force on an Accelerating Car

Problem: A car of mass 1000 kg speeds up from rest to 20 m/s in 8 s on a straight road. Find (a) its acceleration and (b) the net force needed.

Given: m = 1000 kg, u = 0 m/s, v = 20 m/s, t = 8 s

Step 1 — Acceleration: a = (v − u)/t = (20 − 0)/8 = 2.5 m/s²

Step 2 — Net force (Second Law): F = m × a = 1000 × 2.5 = 2500 N

Step 3 — Cross-check using momentum: change in momentum Δp = m(v − u) = 1000 × (20 − 0) = 20000 kg·m/s

Rate of change of momentum = Δp/t = 20000 / 8 = 2500 N — exactly the same answer, as it must be.

Answer: The engine must supply a net forward force of 2500 N, i.e. 2.5 kN.

Newton's Third Law: Action and Reaction

Newton's Third Law says: to every action there is an equal and opposite reaction. Whenever body A pushes body B, body B pushes back on A with an equal-sized force in exactly the opposite direction. When you jump, your legs push the Earth down and the Earth pushes you up. A rocket throws hot gases downward and the gases push the rocket upward. A swimmer pushes water backward and the water drives the swimmer forward. Forces always come in pairs — you can never have a lonely single force. A beautiful consequence is the law of conservation of momentum: when no external force acts on a system, its total momentum stays constant. That is exactly why a gun recoils backward when the bullet shoots forward.

⚠️ Common mistake: students think action and reaction forces cancel each other, so nothing should ever move. They do NOT cancel — because they act on two different bodies. The action acts on B, the reaction acts on A. Only forces acting on the SAME body can be added or cancelled. Mix this up and every Newton's-law problem goes wrong.

🧠 Why You Can Walk

Think about walking on a road. Your foot pushes the ground backward; by the third law, the ground pushes your foot forward — and that forward push is what moves you. On smooth wet marble or ice, the ground cannot push back enough (too little friction), so your foot slips and you can barely move. Every single step you take is Newton's third law and friction quietly working together.

Friction and Free-Body Diagrams

Friction is the force that opposes relative motion (or the tendency of relative motion) between two surfaces in contact. It is why your notebook slides to a stop and why brakes work. Static friction acts when a body is still but on the verge of moving, and it self-adjusts up to a maximum value; kinetic (sliding) friction acts once the body is already moving. To solve any force problem cleanly, we draw a free-body diagram (FBD): a simple sketch of just one chosen object with every force acting ON it drawn as an arrow.

How to Draw a Free-Body Diagram

1Pick ONE object to study and imagine it floating alone — mentally erase everything else.
2Draw its weight (W = mg) as an arrow pointing straight down.
3If it rests on a surface, draw the normal force N perpendicular to that surface (usually pointing up).
4Add any applied push or pull, string tension, and friction (drawn opposite to the motion or the tendency to move).
5Choose x and y axes, then apply ΣF = ma along each axis to write your equations and solve.

Feature	Static friction	Kinetic friction
When it acts	Body at rest but tending to move	Body already sliding
Behaviour	Self-adjusts up to a maximum	Stays roughly constant
Formula	f_s ≤ μ_s N	f_k = μ_k N
Relative size	Its maximum (μ_s N) is larger	Smaller, since μ_k < μ_s

Quick Revision

✓Inertia: every body resists a change in its state of rest or motion, and mass is the measure of inertia.
✓First Law: no net force means no change in velocity (a = 0).
✓Second Law: F = ma = rate of change of momentum; 1 N = 1 kg·m/s².
✓Momentum p = mv is conserved when no external force acts on the system.
✓Third Law: forces occur in equal and opposite pairs that act on two different bodies.
✓Friction opposes relative motion; maximum static friction is greater than kinetic friction (μ_s > μ_k).
✓Always draw a free-body diagram before writing ΣF = ma.

⚡ Quick check

A book lies at rest on a table. Which of these is a correct action–reaction pair under Newton's third law?

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