Hi Guys. I catch myself with confused a little bit of friction and physics. It's really simple to understand if it's a 1 box. And then you know that friction is opposed the motion. But it's little complicated for me in those cases : Case 1 : I assume that we will push the box 1 to right. Then the friction from the ground is to the left. And for the box 2 because of the third law of Newton we will have equal and opposite reaction and the box 1 will exert a force on box 2. So direction on box 2 should be to the left? Is it okay that this force is friction force? Or it's just transfered force from box 1 and then we will have the friction force acting to the right to hold this force? Case 2 : The same question probably here as in case 1. The friction for box two will be to the left, and the box 1 will get equal and opposite reaction to the right because of the pushing with force F. But what about the friction? Is it the 3rd Law force? Where is the direction for force of the ground ? Case 3 and 4 : I have problem with wheels that I don't know when should I look for motion as rotating and then decided the friction direction as opposed for example for Clockwise movment and when I should consider the as a opposed for translation movement. All this case we consider as a Static Problems. ΣF=0. Please help with this confuse. Ps. Can you recommend good book to refresh this fundamentals? I consider the : Collins Advanced Science by Dobson. But I almost sure that it's not so detailed in things like that.