A box with mass M is dragged across a level floor having a coefficient of kinetic friction (greek mu)k by a rope that is pulled upward at an angle Theta above the horizontal with a force of magnitude F. What is the expression of the mag. of force needed to move the box with a constant speed? I dont' really know what to do on this. What I've tried so far is spliting the angle's force into acos(theta) and asin(theta), then somehow applying friction. Does anyone know where I can read up on friction, maybe I just dont' understand the concept fully.