Rotational motion question

[smt]

I'm studying the chapter on rotational motion, and I have a problem that I don't have a clue how to solve, the problem is this: A spool of wire of mass M and radius R is unwound under a constant force F. (F is parallel to the ground) Assuming the spool is a uniform solid cylinder that doesn't slip, show that (a) the acceleration of the center of mass is 4F/3M, and (b) the force of friction is to the right, (same direction as F) and equal in magnitude to F/3.

(b) is the part I really don't have a clue on, can anyone help me solve this problem?

thanks in advance,

smt

 

[DavidWhitbeck]

Well you should show work but it's very simple-- use Newton's 2nd Law to solve (b). You know the acceleration and thus the net force which must be the sum of F and f.