https://www.physicsforums.com/showthread.php?t=259689 In that thread how did the person figure out the contact force between mA and mB? I know they got the acceleration by doing F/mA+mB. But I don't get how they got the contact force, because all the blocks are equal mass. Something like F-a*(10+10/2)???? What are they multiplying the F-a by? Thanks.