I put my hair in a bun yesterday, and it gave me a crick in my neck. I realized a few things.

1) I have a lot of hair. It's almost as long as it was back in college. Coupled with the fact that I have really thick hair, it can be rather heavy.

2) My neck doesn't care so much about the mass of my hair as it does its moment of inertia. I can wear my hair down or in a ponytail and my neck doesn't get sore. I put it up in a bun, and now all the weight is in a 6-inch ball at back of my head, and its effective torque on my neck (mass times distance) is a lot bigger. Imagine holding a bag of groceries with your arm straight down versus extended out from your body.

3) I am a big old physics nerd. I'm so pleased with myself.