1. The problem statement, all variables and given/known data If the coefficient of static friction between the sighn and his hands is 0.6, and the coefficient of static friction between the sign and the wall is also 0.6, with what force must the boy push to keep the sign in place? 2. Relevant equations 0.6(Fn)=100 N Fn=166.7 N 3. The attempt at a solution So my question is this: Why does the boy only need to exert half of the normal force in order for the sign to stay in place? My reasoning is this: he pushes the sign, which pushes the wall, the wall pushes against the sign with the same force that the boy is exerting. But this makes me want to think that there are two normal forces, one from the boy and the wall and one from the sign and the wall. Is this true, or is there another reason the boy pushes with only half the normal force? Thanks!