[SOLVED] Rolling object and friction

[kasse]

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

I want to find an expression for the coefficient of friction when this object starts to skid when we pull it with a force F

http://www.badongo.com/pic/3639015

3. The attempt at a solution

Ff = (mju)*Mg

I found that I can put this into Newton's 2. law for rotation and solve for alpha = 0. But why can't I put it into newton's 2. law for linear motion and solve for alpha = 0 (a = (alpha)*R)?

[Doc Al]

But why can't I put it into newton's 2. law for linear motion and solve for alpha = 0 (a = (alpha)*R)?
You can certainly apply Newton's 2nd law for translational motion. But "a = (alpha)*R" only applies when it rolls without slipping, which is not the case here.