An object resting on a table weighs 100 N. So the object is pushing down on the table with 100N of force.
The table is pushing UP on the object with 100N of force.
There are two reasons for saying this. They both come from Newton's laws of motion:
-- From Newton's 3rd law: For every action, there is an equal and opposite reaction. If the object pushes the table, down, with 100N of force, then the table pushes the object, up, with 100N of force. THERE's your pair of equal, opposite forces, and Newton is happy.
-- From Newton's 2nd law: The net force on an object is equal to the mass of the object multiplied by its acceleration. Imagine a piece of paper slipped in between the object and the table. The paper is not accelerating, so the net force on it must be zero. If the object is pushing down on the paper with 100/N of force, then ... in order for the net force on the paper to be zero ... the table must be pushing up on the paper with 100N of force.
