Final answer:
The friction force acting on the table is equal and opposite to Marie's push when she pushes on it horizontally without causing it to move, illustrating the principle of static friction.
Step-by-step explanation:
If Marie pushes horizontally on a table, but not hard enough to get it moving, the friction force acting on the table is equal and opposite to her push. This is a consequence of Newton's third law, which states that for every action, there is an equal and opposite reaction. Since there is no movement, the friction force must be equal to the applied force to prevent motion, not exceeding it and not falling short. Friction in this scenario is called static friction, which acts to prevent two surfaces from sliding past each other. The magnitude of static friction will adjust up to its maximum value to match the applied force, as long as the applied force does not exceed the maximum static friction threshold.