Suppose that I were to jump on the surface of the Earth. The moment I hit the surface, shouldn't the force Earth is applying on me be greater than the force I am applying to it so that I stop completely? If the forces were equal, wouldn't I still be moving?
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"If the forces were equal, wouldn't I still be moving?"
No: the upward contact force on YOU from the Earth continues to act (with changing magnitude) until you are at rest, in equilibrium, at which stage the upward contact force on you is equal to the Earth's downward gravitational pull on you. (This is the equilibrium condition and nothing to do with Newton's third law.)
The downward contact force that you exert on the Earth is always equal and opposite to the contact force that the Earth exerts on you; these forces are Newton's third law partners in the same interaction. But the force that you exert on the Earth is irrelevant to your motion! It's the forces that act on YOU that affect your motion.
Philip Wood
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