Angular Momentum
Spin a platform up, then pull two hand-held masses in toward the axis and watch the spin rate jump — angular momentum stays fixed the whole time, so pulling the masses in tighter is the only thing driving the speed-up.
Preview — sign up free to change the numbers yourself
What you can adjust
- Starting spin rate
- 0.2 – 2.5 rev/s
- Mass in each hand
- 1 – 8 kg
- Starting arm radius
- 0.5 – 1.1 m
- Final arm radius
- 0.15 – 0.6 m
- Body + platform rotational inertia
- 0.5 – 5 kg*m^2
- Pull-in time
- 0.5 – 5 s
How fast the platform is spinning before the pull-in begins. This sets the total angular momentum (L = I*omega) once, right at the start — everything that follows is the system keeping THAT number fixed.
Mass held in EACH hand. A bigger mass makes the 2*m*r^2 term — the masses' own share of the total rotational inertia — larger, so pulling the same distance in produces a bigger swing in spin rate.
How far out the masses start — arms fully outstretched from the spin axis.
How far in the masses get pulled. Rotational inertia depends on radius SQUARED, so pulling this in to half the starting radius doesn't halve that part of the inertia — it quarters it, which is why the spin-up looks so dramatic.
The rotational inertia of the person's own body and the platform itself, fixed for the whole run. Only the masses' own 2*m*r^2 contribution changes as the arms move.
How long the pull-in takes (the pull back out afterward mirrors it exactly). This is pure pacing — L and I depend only on WHERE the masses are, never on how fast they got there — but a very short time makes the spin-up hard to watch.
About this simulation
What it shows
A person on a spinning platform holds a mass in each hand and pulls them in towards the axis. The spin rate jumps as the arms come in.
The key idea
With no outside turning force, angular momentum (rotational inertia × spin rate) stays fixed. Pulling the masses in lowers the inertia, so the spin rate has to rise.
Try this
Lower Final arm radius from 0.3 to 0.15 m: the masses end up closer to the axis, so the spin rate finishes higher.