Spin chair: pull in, spin up
"Spin chair: pull in, spin up" is a free, interactive Class 11 Physics simulation for NEET and JEE, built on the lesson "Angular momentum and its conservation" in System of Particles and Rotational Motion. Press Play, then move her hands in and out. Nothing outside twists the chair, so her angular momentum L = Iω stays 9.6 kg m²/s. With no external torque, I₁ω₁ = I₂ω₂. Pulling mass towards the axis lowers I and raises ω; the kinetic energy L²/2I rises, paid for by the work of the muscles. JEE Main asked 66 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Angular momentum and its conservation in System of Particles and Rotational Motion. Change the values and watch what happens.
In JEE Main
42 shifts analysed
66 questions from System of Particles and Rotational Motion across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT §6.12, §6.12.1
- Take one particle at distance r⊥ from the fixed axis: the part of its angular momentum along the axis is mr⊥²ω. Adding over the body gives L_z = Iω.
- For a particle, l need not be along the axis: l and ω are not necessarily parallel, unlike p and v, which always are.
- If the axis is a symmetry axis of the body, the perpendicular parts of the particles' angular momenta cancel in pairs, and L = Iω exactly along the axis.
- For a fixed axis, dL_z/dt = τ along the axis, while the component of L perpendicular to the axis stays constant. With I constant this gives τ = Iα again.
- Conservation: if the external torque about the axis is zero, Iω = constant. If I decreases, ω increases in the same ratio, and the other way round.
- A person on a frictionless swivel chair spinning with arms folded slows down on stretching the arms out (I increases) and speeds up again on pulling them in.
- Acrobats, divers, skaters and dancers doing a pirouette on the toes of one foot use this: pulling the arms and legs in reduces I and raises the spin rate.
- Kinetic energy ½Iω² = L²/2I is not conserved when I changes: pulling the arms in raises it, and the extra energy comes from the work done by the person's muscles.
Questions
3 answers
- What does the "Spin chair: pull in, spin up" simulation show?
- Press Play, then move her hands in and out. Nothing outside twists the chair, so her angular momentum L = Iω stays 9.6 kg m²/s.
- Does JEE Main ask questions from System of Particles and Rotational Motion?
- Yes. JEE Main asked 66 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Angular momentum and its conservation" in System of Particles and Rotational Motion (Class 11 Physics).
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