Sunday, January 30

Blog 9: Another xkcd-based blog

So I've been following xkcd for a while, and when we got to our section on angular momentum, I finally understood one of the comics!

It's got all the nerdiness of angular momentum combined with all the romance of throwing up due to nausea! Nice.

Actually, the physics here is (are?) sound enough. Angular momentum of a system (in this case, the girl and the Earth) is  constant. That means if the girl spins in the counterclockwise direction, the Earth will spin more in the clockwise direction. Time travel ahoy!

Hardly. If that worked, any time we needed more time to cram for tests, the school would be filled with kids spinning. Why doesn't it work?

Angular momentum= moment of inertia* angular velocity

Moment of inertia= mass* lever arm distance* lever arm distance. (Actually, the equation for moment of inertia varies depending on the distribution of mass, but go with me here.) The girl's mass is going to be around 60 kg, but the Earth's mass? More around 5.9 followed by twenty-three zeroes. Pretty big difference.

Who needs skin?
The Earth's huge mass means the change in angular velocity caused by the girl would be negligible. She could probably spin so quickly that her skin ripped off her body without having a discernible impact on Earth's rotation.

Clearly this girl didn't  hear, or maybe she doesn't care. Either way, she should put her arms out to increase her moment of inertia. Some bowling pins would not be amiss, either.