The experimenter fom the video rotates on his stool, this time holding his empty hands in his lap. You stand on a desk above him and drop a long, heavy been bag straight down so that it lands across his lap, in his hands. what happens?

The experimenter is rotating on his stool with angular velocity ω (suppose)

His moment of inertia is I say

We are applying no torque from outside. therefore, the angular momentum will remain the same

Thus angular momentum L = I ω = constant

Thus we can say I₁ ω₁ = I₂ω₂ = constant

here I₁ is the initial moment of inertia and ω₁ is the initial angular velocity

Similarly I₂ is the final moment of inertia and ω₂ is the final angular velocity

When a been bag is dropped on his lap, his moment of inertia increases due to increase in mass

In the above equation, when moment of inertia increases, the angular velocity decreases. So its motion of rotation will decrease.