Below is a problem I had on a test recently. The task is to find out what the friction coefficient must be in order to prevent the big box from moving. Friction between the little box and the slope is neglected.The red markings are my ideas. I figured if I sum the forces in the x-direction I'll find the friction force, which I can replace by uN, since F=uN. Will N be the same as G? If so, finding u is easy...but I think it's more complex than that, can someone explain?