# Rolling resistance and static friction

I am a bit confused about the relation between rolling resistance and static friction.

I have often heard that it is the static friction that lets the wheel roll. Consider the following two cases:

a) A tire of a car of mass $m$ moving with positive acceleration $a$ on a street
b) A wheel of a moving wagon (of mass $m$) which is pulled by a string (with force $F_\mathrm{string}$).

1. How do the free body diagrams look like? (including static friction, rolling friction and friction forces at the axis)
2. What is the resulting force accelerating the center of mass of the wheel in case b) (in dependence of the frictional coefficients and $m$)
-
Please make your question more clear. Is it "What is the difference between rolling friction and static friction on a wheel?" or something else? Like this it is looks too much like a do-my-homework question. –  Maksim Zholudev Jan 9 '12 at 9:17