Hi, Please take a look at this figure: Screenshot by Lightshot There is a force that is applied so that block B will not fall under the influence of gravity. I have set up the problem as: F= (ma+mb)a μR=mbg where μ is the coefficient of friction and R is the reaction force. Now my question is I can't figure out why mba is the reaction force. What exactly is maa and mbb? Shouldn't maa be the force of block a on b? Then what about mbb? What is this force? The force direction should be pointing to the right side (since the applied force is to the right)? So it is as if mbb is acting on thin air? I can't wrap my head around what is maa and mbb, please help. Thanks.