Hi, The below questions are NOT HW questions, but I have a big exam coming and I would wholeheartedly appreciate some/any assistance with the following two issues, namely: (1) Direction of static friction in case of rolloing without slipping (2) Direction of α and VCM in case of rolling without slipping (1) Let us please examine the three diagrams attached: Case 1, showing a sloped plane, the cylinder is rolling down the plane, clockwise, and the static friction is causing a moment (with respect to the center), equal to fR (-z) causing it to roll. Here, the static friction is aiding the rolling. Case 2, showing a cylinder let's say, rolling without slipping, clockwise, whilst the (static?) friction, in the direction as delineated in the diagarm (I didn't draw it!), is creating a moment opposing that of the tension, causing the cylinder to slow down. Here, the friction is resisting the rolling. Case 3, supposing the cylinder is rolling at a given moment to the left (i.e. counterclockwise), which was defined to be the positive X direction, the static friction acting on the cylinder at the point of contact with the bar, would be to the right. Thus, the static friction is, again, aiding the rolling. Now comes my question - based on these three examples, I see that at times the static friction aids the rolling, whereas it could, on other occasions, resist it (case 2). How may I then determine its direction? How may I know whether it is to assist the rolling or act against it?? (2) I have seen that, at times, VCM = -ωR (instead of ωR), in rolling without slipping. Why is that and, most importantly, how may I determine the correct sign for my equations??