Also, why do you spin faster when you bring your arms in?
In: Physics
’Rotational force’ is a little strange wording. Generally rotation will be created by any force which has a moment arm, which will give it a moment. The rotation such a moment can give to an object depends on the mass of that object and how far from the centre of gravity that mass is located. This is a reflected in what’s called the polar moment of inertia.
If no external moments are acting on a an object, that object’s angular momentum (which is dependent on angular velocity and the polar moment of inertia) will stay the same. So if you bring your arms in, your mass will be closer to your centre of gravity, so your polar moment of inertia will go down. Your angular momentum stays the same, so your angular velocity will increase (which means your start spinning faster).
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