JEE/NEET Physics · Laws of Motion series · Part 1 of 8 · All parts →
- First law: no net force → velocity never changes (rest stays rest, motion stays motion)
- Second law: F_net = ma — net force is the ONLY producer of acceleration
- Third law: forces come in pairs, equal and opposite, on DIFFERENT bodies
- The FBD: draw ONE body, draw only the forces ON it — the method that solves everything
- Inertia is mass’s resistance to velocity-change: more mass, same force, less acceleration
Everything you’ll ever study in mechanics is contained in three sentences a 37-year-old Englishman wrote in 1687. Missiles, muscles, moon missions — all consequences of Newton’s three laws. Part 1 of the Laws of Motion series — the highest-weightage chapter of Class 11.
- The first law: the law of laziness
- The second law: the cause of change
- The third law: the law of pairs
- The FBD: physics’s single best tool
- Solved examples
- Common mistakes
- This physics in your daily life
- Practice set
- Recap
The First Law: The Law of Laziness
Things don’t like change. With zero net force, a resting object rests forever, and a moving object cruises at constant velocity forever. Motion needs no cause — only changes in motion do. This overturned 2000 years of Aristotle (‘things need pushes to keep moving’) — they don’t; friction just hides the truth.
The Second Law: The Cause of Change
| Letter | What it means (plain words) | Value / unit |
|---|---|---|
| F_net | the VECTOR SUM of all forces on the body | newtons (N) |
| m | the mass — the laziness rating | kg |
| a | the acceleration produced | m/s², same direction as F_net |
Double the net force → double the acceleration. Double the mass → half the acceleration. It’s a recipe with three ingredients, and it’s why mass is called inertia: the measure of a body’s refusal to change its velocity.
The Third Law: The Law of Pairs
You push the wall; the wall pushes you back, equally hard, oppositely directed — and crucially, on different bodies. That’s why the pair doesn’t cancel: they act on different victims. Walk, swim, rocket-launch: every ‘push off something’ in the universe is third law.
The FBD: Physics’s Single Best Tool
The free-body diagram isolates ONE body, strips everything else away, and draws only the arrows of force acting ON it. Weight down, normal up, tension along the rope, friction along the surface. Then F_net = ma per axis. Every problem in this series begins with an FBD — no exceptions.
Solved Examples
a = F/m = 20/5 = 4 m/s².
✔
Answer: 4 m/s²
F_net = 10 − 4 = 6 N → a = 6/2 = 3 m/s² right.
Only the NET force accelerates — the individual forces merely argue. ✔
Answer: 3 m/s²
Man: N (platform’s push up) − 600 = 0 → N = 600 N.
By the third law the man pushes the platform DOWN with 600 N too — same pair, different bodies.
600 N — the pair never cancels because it never acts on one body. ✔
Answer: 600 N (action-reaction pair)
- ‘Motion needs a force.’ No — CHANGE of motion needs a force; constant velocity is free (first law).
- Adding action-reaction pairs into one FBD. The pair acts on the OTHER body — your FBD contains only forces on YOUR body.
- Using F = ma with any single force. Only the net force enters; subtract opposing forces first.
- Weight vs mass confusion. Mass (kg) is laziness; weight (mg, newtons) is gravity’s pull on it — related but different animals.
This Physics in Your Daily Life
- Seatbelt versus windshield — the car stops; your body obeys the first law and keeps going at 50 km/h until something supplies the stopping force. The belt volunteers.
- You ‘fall forward’ when the bus brakes — no mystery force: your body simply continued, per the first law, while the bus slowed beneath it.
- Every step you take — you push the Earth backwards; the Earth pushes you forwards (third law). You move; the Earth’s change is unmeasurably small (its mass is unmeasurably big).
- Rockets work in empty space — nothing to push against? They push their own exhaust backwards; exhaust pushes them forwards. Third law needs no ground.
- Shopping-trolley physics — a full trolley accelerates lazily, an empty one zooms: same push, different mass, F = ma at the supermarket.
Aristotle thought a moving arrow needed a continuous push. Galileo stared at ice pucks and suspected laziness instead; Newton wrote it as law: the universe’s default is ‘keep doing whatever you’re doing’. Pushes don’t cause motion — they cause CHANGES. The arrow flies because nothing stops it yet.
Same 100 N push on a 50 kg cart and a 1000 kg car: a = 2 m/s² versus 0.1 m/s² — same force, tenfold difference in obedience. And a 1000 kg car braking at 5 m/s² needs exactly F = 5000 N of friction, always, no exceptions: the recipe never lies.
Draw the FBD: the body as a dot, arrows radiating outward — weight down, normal up, applied pushes, friction opposing. Circle the net arrow per axis. Every mechanics problem, from pencil to spacecraft, is this same picture with different arrow labels.
Practice set (answers hidden — try first)
(NEET-level) 10 N on 4 kg (frictionless): a =
(JEE Main-level) Net of 15 N right, 5 N left on 2 kg: a =
(NEET-level) A body moves at constant velocity. Net force =
(Concept) Action and reaction cancel each other when:
(JEE Main-level) 2 kg at 3 m/s² needs net force:
- 1st: no net force → velocity constant forever
- 2nd: F_net = ma, same direction
- 3rd: pairs act on different bodies — never cancel
- FBD: one body, only forces ON it
- mass = inertia = resistance to change
- 🔁 three laws in one line each
- 🔁 F_net = ma per axis
- 🔁 action-reaction on different bodies
- 🧠 Chant: ‘lazy, recipe, pairs’.
- 🧠 FBD rule: ‘one body, all arrows in, nothing else’.
- 🏠 Daily: bus-brake lurch = first law in your face.
- 🏠 Daily: walking is pushing the Earth backwards.
Quick revision
- First law: no net force → velocity never changes (rest stays rest, motion stays motion)
- Second law: F_net = ma — net force is the ONLY producer of acceleration
- Third law: forces come in pairs, equal and opposite, on DIFFERENT bodies
- The FBD: draw ONE body, draw only the forces ON it — the method that solves everything
- Inertia is mass’s resistance to velocity-change: more mass, same force, less acceleration
- The first law: the law of laziness
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