Final answer:
The desk exerts an equal and opposite force of 4.9 N against the notebook due to Newton's third law, indicating an equilibrium state with balanced forces.
Step-by-step explanation:
If a notebook on a desk has a weight of 4.9 N, then according to Newton's third law of motion, which states that for every action there is an equal and opposite reaction, the magnitude of the force with which the desk pushes back on the notebook is also 4.9 N. This opposite force is known as the normal force, and it is the force exerted by a surface to support the weight of an object resting on it without acceleration. The notebook is in a state of equilibrium, where all forces balance, meaning that the upward force of the desk must exactly match the downward force of gravity on the notebook.