The acceleration of the man is due to the gravitational force exerted
by the earth, and has a magnitude given by
F = ma = (50)(9.8) = 490
N. We
now have the force on the man, but we need to find the force exerted on
the earth. To do so, we simply invoke Newton's third law. If the earth
exerts a force on the man directed toward the center of earth, the man
exerts a force on the earth equal in magnitude, but opposite direction.
Thus the earth experiences a force of 490 N directed away from its
center. This force, like all other forces, causes the earth to
accelerate. This seems counterintuitive, as the earth seems to be a
stable object. The acceleration caused, though, is quite small due to
the large mass of the earth, and is negligible.
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