* Newton's Third Law: For every action, there is an equal and opposite reaction. This means the force the man exerts on the Earth is *exactly the same* as the force the Earth exerts on him.

* Weight vs. Force: The man's weight is 800N. This is the force of gravity acting on him due to the Earth's mass. He doesn't "exert" this force; it's a force *acting on him*.

Therefore, the man exerts a force of 800N on the Earth.

However, because the Earth is so much more massive than the man, the effect of his force is negligible. The Earth's motion isn't noticeably affected by him standing on it.