# Homework Help: Dynamics: Rotating collar

1. Mar 2, 2016

### kspabo

1. The problem statement, all variables and given/known data

2. Relevant equations

Radial and Transverse coordinates to relate acceleration

Balancing forces

3. The attempt at a solution

I know that there is more to this problem, but my question is simple: Why don't we know what the normal force is? When I solved it myself I just set the Normal force to mass * gravity... why isn't this the case for this problem? All of the solutions I've found have them creating a relation between the X and Y force eqns. and setting them equal to solve. Why can't we just set N = mg as I was so used to doing?

2. Mar 3, 2016

### Nidum

You haven't told us what problem you are trying to solve . Please post the original statement of the problem as given to you .

3. Mar 3, 2016

### kspabo

My apologies, I completely forgot to include that part. Here it is:

4. Mar 3, 2016

### BvU

N can only work in a normal direction (as the word normal force suggests ) . Without friction, the only direction in which the rod can exercise force on the collar is perpendicular to the rod.
You have figured out the components of N, so you can add them up (vectorially) to get $|\vec F_N|$.

 no, sorry, you needed to calculate that |F| from its vertical component to get r.