Re: How do you tell left and right to an alien, revisited

Tony,

I think the key to this question is the concept of conservation of angular momentum. You're absolutely right in that the angular momentum vector is defined by the right hand rule, but you may be forgetting that *change* in angular momentum results in torque. The reason a person (or alien) will spin to the left in the chair is to balance out the wheel spinning to the right.

In mathematical terms: If we initially have a system with angular momentum = L_0, the system better have the same angular momentum even after we rotate the wheel by 180 degrees (thanks to conservation of angular momentum). Therefore, we will have:

L_0 = L_alien + L_wheel.

And since the wheel was rotated 180 degrees, we know that L_wheel = -L_0. If you plug this into the above equation, you get:

L_0 = L_alien - L_0.
Therefore: 2*L_0 = L_alien.

So if the alien has an angular momentum of twice the original momentum, which was pointing up, by the right hand rule, the alien will spin counterclockwise, or to the left.

For further reference, try Serway's "Physics for Scientists and Engineers"

Hope that helps clear things up for you,
Amber