as many of you know I sit in chairs... a lot. And given the new information we have learned in physics class I can analyze the types of forces acting on me while I sit. Gravity being a constant is always acting and will continuously act on me as MG. Another force acting on me is the normal force -- this is the perpendicular force the sight of contact applies. The normal force must be equivalent to MG because I am not moving and have no acceleration leading me to the conclusion that I am in equilibrium. But lets mix it up a little bit and say someone really wants the seat im sitting in and decides its a good idea to push me. Unless there was someone else too push me in the opposite direction I would be knocked out of equilibrium and fly off my chair. And that is the physics of sitting in a chair.