If two objects interact, the force `vec(F)_(12)` exerted by object 1 on object 2 is equal in magnitude and opposite in direction to the force `vec(F)_(21)` exerted by object 2 on object 1: `vec(n)`: Normal Force `vec(F)_(tm)`: Force exerted by table to monitor. `vec(F)_(mt)`: Force exerted by monitor to table. `vec(F)_g` : gravitational force. `vec(F)_(Em)` Force exerted by Earth to monitor. `vec(F)_(mE)` Force exerted by monitor to Earth. Figure a and b: `vec(F)_(Em) = -vec(F)_(mE)` where `vec(F)_(Em) = vec(F)_g` `vec(F)_(tm) = - vec(F)_(mt)` where `vec(F)_(tm) = vec(n)` Figure c: A free- body diagram shows the monitor as a black dot with the forces acting on it. In a free-body diagram, the particle model is used by representing the object as a dot and showing the forces that act on the object as being applied to the dot. When analyzing an object subject to forces, we are interested in the net force acting on one object, which we will model as a particle.