JEE/NEET Physics · Rotational Motion series · Part 5 of 8 · All parts →
- Spin quantity L = Iω (laziness × spin rate) — or mvr for a single moving mass
- No outside turning power → spin quantity NEVER changes
- Pull mass inward: laziness drops → spin rate rises (the skater)
- In spin collisions, L survives even when energy crashes
- Hidden work: muscles or motors pay for any speed-up at constant L
The skater pulls her arms in and doubles her spin — you met the idea in Gravitation. Now we put numbers on it, and meet the strangest collisions in physics: ones where energy vanishes but spin quantity survives untouched. Part 5 of the Rotational Motion series.
- Spin quantity, simply
- What each letter means
- The unbreakable rule, with numbers
- Spin collisions: where energy dies but L survives
- The hidden work
- Solved examples
- Common mistakes
- This physics in your daily life
- Practice set
- Recap
Spin Quantity, Simply
Every spinning thing carries a ‘spin quantity’ — how much turning it has. Two ways to count it: a rigid body spinning: laziness × spin rate; a single mass going around a point: mass × speed × distance (Gravitation Part 4’s L = mvr). Same quantity, two costumes.
What Each Letter Means
| Letter | What it means (plain words) | Value / unit |
|---|---|---|
| L | spin quantity (angular momentum) | kg·m²/s |
| I | spin-laziness about the axis | kg·m² |
| ω (omega) | spin rate | rad/s |
| τ (tau) | outside turning power (torque) | N·m — the only thing that can change L |
The Unbreakable Rule, With Numbers
No outside turning power → L never changes. Skater: arms out, I = 6 kg·m², ω = 2 rounds/s. Arms in: I = 3. Locked L: 6 × 2 = 3 × ω’ → ω’ = 4 rounds/s. Doubled spin, zero pushing — the speed-up is pure bookkeeping. Arms out again: back to 2. The see-saw: laziness down ⇄ spin up, always.
Spin Collisions: Energy Dies, L Survives
When things spinning collide and stick — a bullet embedding in a door, a child landing on a merry-go-round — the impact is so brief that outside turning can’t matter. So spin quantity before = spin quantity after. But energy? Wrecked — heat, denting, sound. Two separate ledgers: L survives the crash; energy usually doesn’t.
The Hidden Work
Halve the laziness at constant L and the spin energy doubles (energy = L²/2I). Nobody gave it for free — the skater’s muscles did work pulling her arms in against the ‘outward fling’. Whenever spin rate rises at constant L, somebody paid. Find who.
Solved Examples
Lock L: 6×2 = 3×ω’ → ω’ = 4 rad/s.
Energy = L²/2I: halving I doubles energy — muscles paid. ✔
Answer: ω’ = 4 rad/s; spin energy doubles
Lock L about the axle (axle forces pass through it — no turning):
Before: I = ½MR² = 200; L = 400.
After: I = 200 + 20×2² = 280 → ω’ = 400/280 ≈ 1.43 rad/s.
Check: more laziness at locked L = slower — ✔ Energy dropped too: the landing was a crash (stuck together), so energy legitimately died.
Answer: ω’ ≈ 1.43 rad/s
Why L: the crash is instant, and the hinge’s forces pass through the hinge — zero turning about it.
Before: bullet’s spin quantity = mvr = 0.01 × 400 × 1 = 4.
After: (door laziness ML²/3 = 4, plus bullet 0.01×1²) × ω = 4.01ω.
ω = 4/4.01 ≈ 1 rad/s.
Energy audit: bullet arrived with 800 J; the door+bullet now carry ~2 J — 99.7% became heat and dent. L survived; energy didn’t. ✔
Answer: ω ≈ 1 rad/s (and 99.7% of the energy died)
- ‘L is always conserved.’ Only when outside turning power is zero about your chosen axis. A spinning disc on a rough table bleeds L through friction’s turning power.
- Saving energy along with L in crashes. Sticking collisions destroy energy while preserving L. Assuming both gives unsolvable or wrong equations.
- Using Iω for a single mass. A lone bullet has mvr; Iω is for rigid bodies on a defined axis.
- Wrong axis choice. The bullet-door problem conserves L about the HINGE (forces pass through it) — about the door’s middle, they don’t.
This Physics in Your Daily Life
- Pulsars: a dying star collapses from Earth-size to 10 km — laziness collapses a million-fold, so spin explodes to hundreds of rounds per second. The skater’s trick, violently, at cosmic scale.
- Divers and aerial skiers tuck to somersault fast, stretch to slow for entry — every twist you’ve applauded was this rule.
- Chandrayaan-class spacecraft steer with reaction wheels: spin a wheel inside one way, the whole craft turns the other — L shuffles internally, total unchanged, no fuel.
- Helicopters need tail rotors: the engine spins the main blades one way; the rule spins the body the other. The tail rotor cancels it — the most visible conservation law in the sky.
- Hard drives and fans coast for seconds after power-off — no turning power, spin quantity drains only slowly through tiny friction.
Practice set (answers hidden — try first)
(NEET-level) Skater halves her laziness at constant L. Spin rate:
(JEE Main-level) L = 10 kg·m²/s, I = 2 kg·m². Spin energy:
(Concept) A spinning disc dropped on a rough table:
(JEE Main-level) A child walks from rim to centre of a free roundabout. Spin rate:
(Concept) In the bullet-door crash, why conserve L about the hinge?
- 🧠 Skater chant: ‘arms in = spin up, arms out = spin down — nobody pushed’. Bookkeeping, not muscle.
- 🧠 Crash rule: ‘L survives, energy dies’ — sticking collisions preserve turning, wreck energy.
- 🧠 Energy at locked L = L²/2I — whoever changed the laziness PAID. Find who.
- 🏠 Daily: a hard drive coasts seconds after power-off — locked turning quantity draining slowly.
- 🏠 Daily: spacecraft turn with internal wheels (Chandrayaan-style) — spin a wheel, the craft counter-turns, zero fuel.
- L = Iω (rigid body); L = mvr (single mass)
- no outside turning → L locked, whatever happens inside
- see-saw: laziness down ⇄ spin up (arms in = faster)
- sticking collisions: L survives, energy dies
- energy at constant L = L²/2I — whoever changed the laziness paid
Quick revision
- Spin quantity L = Iω (laziness × spin rate) — or mvr for a single moving mass
- No outside turning power → spin quantity NEVER changes
- Pull mass inward: laziness drops → spin rate rises (the skater)
- In spin collisions, L survives even when energy crashes
- Hidden work: muscles or motors pay for any speed-up at constant L
- The unbreakable rule, with numbers
- 1Centre of Mass: The Point That Behaves Like a Particle
- 2Torque: Why Doorknobs Live Far From Hinges
- 3Moment of Inertia: Rotational Mass, and Why Distribution Beats Size
- 4Torque Equals I-Alpha: Newton’s Second Law, Spun
- 5Angular Momentum in Rotation: Conservation Unleashed
- 6Rolling Motion: Translation and Rotation in One Body
- 7Rotational Energy and Flywheels: Spin as a Battery
- 8Equilibrium and Toppling: Why Cranes Don’t Fall Over
Have a doubt on this topic?
