Newton's Third Law states that forces come in pairs.  Whenever there is a force acting on object A due to object B, there is an equal and opposite force on object B due to object A.   When this vector relationship is expressed in terms of components, we have

• FAB_x = -FBA_x
• FAB_y = -FBA_y

For instance, if a man exerts an applied force on a block of 1 N rightward, then the block exerts a  force on the man of 1 N leftward.