Just some quick notes about rolling without slipping.  For my own clarification purposes.

Rolling without Slipping:




The above site describes rolling without slipping as the sum of the linear component and the rotational component of velocity of the wheel.  At the point of contact between the wheel and the ground, the linear and rotational components are equal and opposite, thus cancelling each other out.  So in the case where they are not the same, rolling with slip occurs.

Rolling with Slipping: http://surendranath.tripod.com/Applets/Dynamics/Roll/SlipRollApplet.html