# Angular momentum

1. Dec 10, 2007

### JolleJ

1. The problem statement, all variables and given/known data
I'm trying to describe the physical phenomena precession using vectors. I need to explain, why a bicycle wheel, which is spinning in the x-y-plane, and only held up in one of the sides, will start do a spin in the x-z-plane.

2. Relevant equations
Using Angular Momentum and Torque; explain the phenomena precession.

3. The attempt at a solution
I understand that gravity and the position vector together creates a torque. This of course change the angular momentum over time. With a simple spin, the angular momentum points along the x-axis. The torque creates a change in the angular momentum along the z-axis. This results in a total angular momentum, which is the sum of the original and the change of angular momentum. This of course changes things a bit. And I can see, why making the wheel turn, so that it is orthogonal on the angular momentum i a solution. However, I cannot see, why this is the only solution to the problem? Why is this exactly what happens? I mean, the angular momentum is defined as a cross product, so there must an indefinite amount of solutions to problem; so why is it that it is exactly the solution, where the wheel turns that is the one that happens?