String wraped around the cylinder, friction?

[ldv]

Homework Statement

we have a cylinder, mass m, radius R. a string is wrapped around the cylinder. and it is all located on the horizontal surface, and when we pull the string, the cylinder starts to roll.
if we pull the string with the constant force F, determine the force of friction (direction and number=mangitude?)

Homework Equations

?

The Attempt at a Solution

well I think that it will start to roll backwards (opposite to the direction of pulling); and so the friction will be in the direciton of pulling. if the force is constant the acceleration is constant. by pulling the string we rotate the cylinder. I don't know how this rotation relates to friction. Is the force of friction proportional to the angular velocity of the cylinder? ie. proportinal to the angular acceleration? ie. the faster we pull the string the faster the cylinder rotates and the friction is bigger. if the cylinder rotates then it has moment of force M=I*alpha, and it is equal to he sum of the moment of force of friction and mom. of force due to deformation of the surface and the body (it says something like that in my book?)

so, can anybody help me, please I and my colleagues are desperate. I must admit that I haven't learned the theory yet, but I will soon, and then I will see that some of things I just wrote don't make sense and that I must have appeared to be an idiot for not knowing how to solve something as simple as this, but till then, can anybody clarify this problem?
tnx in advance

 

[tiny-tim]

Hi ldv!

Hint: start by finding the angular acceleration.