This question comes from Thomas and many other people, who all asked: What would happen if everyone on earth stood as close to each other as they could, jumped, and landed on the ground all at the same instant?
This is one of the most popular questions submitted to What If. It's been examined before, including by a ScienceBlogs post and a Straight Dope article.
They cover the physics pretty well. However, they don't tell the whole story.
At the start of the scenario, the entirety of Earth's population has been magically transported together into one place.
This crowd takes up an area the size of Rhode Island. In fact, let's assume they – I mean, we – are actually in Rhode Island. At the stroke of noon, everyone jumps.
As discussed elsewhere, the jump doesn't really affect the planet. Earth outweighs us by a factor of over ten trillion.
On average, we humans can vertically jump maybe half a meter – and that's when we're not shoulder to shoulder in the middle of a crowd.
Even if everyone did jump that high, and the ground were rigid and responded instantly, the Earth would still only be pushed down by less than an atom's width. Next, everyone falls back to the ground.
Technically, this delivers a lot of energy into the Earth, but it's spread out over a large enough area that it doesn't do much more than leave footprints in a lot of gardens.
A slight pulse of pressure spreads through the North American continental crust and dissipates with little effect beyond moving the needle of a few local seismometers.
