Your answer:
zero

That's correct. If you were exerting any net force on the Earth, then by Newton's Third Law the Earth must be exerting a net force on you, which (by the First and Second Laws, would cause you to accelerate. Since you are at rest (if we ignore small effects due to the Earth's rotation) there must not be any net force acting on you, and therefore your net force on the Earth must also be zero.

This is the only correct answer in the context of Newtonian Mechanics, although many people, including quite a few First Year Physics instructors, find it intuitively difficult to accept. This may be because the gravitational force we exert on the Earth is not "felt" by our haptic senses in the same way as the normal force we exert with our feet on the ground. (We don't use our muscles to exert the gravitational force.)

In the context of General Relativity, however, the second answer (about 700 N) is correct, because in GR gravity is not really a force. Don't ask me to explain GR, but there is an important lesson here about the role of context in Physics. Sometimes the right answer depends on how you look at it.

If you thought this question was "trivial" when you first saw it, what do you think now?

Which (if any) of the "extra" explanations were helpful to your understanding of this question?


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