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quote:Originally posted by fishytailsI'm beginning to learn something i think! 'for every action there is an equal and opposite reaction'so when someone jumps up they exert an equal 'force' downwards as they do going upwards, and the same for when they land again, so there would be no net gain of force in any direction. therefore the law F=ma means that the person accelerates a lot, because of their small mass, while the earth barely moves, due to its enormous mass. the two forces exerted on both the earth and the person must be the same (equal and opposite).it would only work if something hit the earth that hadn't actually propelled itself from the surface beforehand, or maybe if something jumped but never landed?