Spinning in a Chair Physics Demo Explained | It's More Than Angular Momentum

The demonstration of a person sitting in a rotating chair holding two masses out to their sides is a popular demo in physics courses. When the masses are pulled inward, the person spins faster.
The rotational acceleration of the spinning person is often explained as a conservation of angular momentum. Angular momentum is useful in calculating how much the person speeds up, but it does a horrible job of explaining why the person speeds up.
The point of this video is to look at not only how to mathematically model the rotational motion of the person spinning in the chair, it is also to look at the underlying physics which often gets overlooked in this problem. The subjects of centripetal force, work, conservation of energy and linear acceleration are often left out of the explanation of this problem, so in this video we take a deep dive into what's really going on as a spinning person pulls their arms in.
This is a popular demonstration in STEM courses such as high school and college physics. It has appeared on the AP Physics 1 and AP Physics C Mechanics exams and probably has shown up on A Level Physics as well as the JEE.

[@nadsongomes823]

Extremely enlightening. These simpler and intuitive explanations are the best for comprehension of physics.

[@INTEGRALPHYSICS]

Glad you like them!

[@sounaknag4033]

Very intuitive explanation! Thanks!

[@DhenerAbade]

First time that this phenomenom really “clicks” for me. Great job! Thanks

[@3.5_to_5.5]

Great video thanks!

[@INTEGRALPHYSICS]

Glad you liked it!

[@mirabdulsamad5171]

Sir but when we push mass outward we again do work. But the speed decreases instead of increasing?

[@INTEGRALPHYSICS]

True, however; To keep the masses moving in a circle, you must always be applying a centripetal force (inward) so when you allow the masses to move outward to a larger radius, you are actually doing NEGATIVE work because your force (inward) is in the opposite direction of of the masses displacement (outward).

[@mirabdulsamad5171]

@@INTEGRALPHYSICS Sir you are truly fabulous. I am appearing in jee advanced . What should I do for MIT admission for PhD physics

[@INTEGRALPHYSICS]

Impressive. That is well beyond any level of school I have experience with... I wish you luck.

[@mycolourpallet_]

​@@mirabdulsamad5171please let me know how you prepared for it. Thanks

[@mirabdulsamad5171]

@@mycolourpallet_ use lectures from iit professors. And try to imagine physics..

[@tmoo8779]

This is awesome

[@INTEGRALPHYSICS]

Thanks. It was fun to do.

[@tmoo8779]

@@INTEGRALPHYSICS I did it with my office chair and some weights, and it was fun! But I spun for too long and got severely motion sick 😂

[@INTEGRALPHYSICS]

lol

[@JETSson420]

So would this mean if the moon began to get closer to the earth this same phenomenon would occur?

[@INTEGRALPHYSICS]

IRL this is gradually happening as the moon slowly gets further from the Earth, an Earth day gets longer.

[@erinbentson6682]

I'm dizzy just watching! 🤣

[@everestmacdonald7423]

😂😂😂

[@ElmoRitter]

How much does one need to already know to intuit this lol