I love this question. It gets brought up in my classroom every year.
One thing to remember. Mother Nature always balances her books. Newton's 1st Law tells us that for every action, there is an equal and opposite reaction.
Keeping that in mind, lets look at your question...and some of the answers that have been given. Lots of people have pointed out that the mass of the Earth is so much greater than the mass of the entire human population. They are right...especially if you spread the whole population out evenly, you are going to have little or no effect.
Now...let's pretend that you could gather everyone up...the mass of the entire population of the Earth over 100 square miles. Sounds cozy, doesn't it? Now, you have everyone jumping up, and then landing. When you jump, you have to push against the Earth, right? And the Earth presses back, moving ever so slightly toward you hanging in the air. When you land, your mass presses against the Earth, moving it back into position. Keeping in mind, if it moved at all, we are talking about micrometers...but since Mr. Newton provided us with his action and reaction formulas, we know that our action will result in a reaction, leaving a net result of 0.
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