What would happen if everyone on Earth went to the same place and jumped?
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From the brains behind Brains On, this is the Moment of Um. Moment of Um comes to you from APM Studios. I'm Anna Goldfield. When I was a kid, I really liked thinking about what-ifs, you know, imaginary or impossible ideas. Like, what if there was a cow who made chocolate milk? Would she have to eat chocolate grass? If she jumped on a cow trampoline in winter, would she make chocolate ice cream?

How high could you bounce on a cow-sized trampoline? And so on. Our listeners have a lot of what-if questions too, like Dax, who asked, what would happen if everyone on Earth went to the same place? And then, what if they all jumped in the air at the same time? Well, let's see if we can get the scoop from an expert.

So what would happen if everyone on Earth went to the same place and jumped at the same time? My name is Dr. Kylie Kennedy, and I am a physicist at Princeton University, where I do research on the tiniest particles in the universe. I'm really interested in understanding what the building blocks of the universe are and how they interact to form the world we know today.

So when I think about this problem, I want to paint the picture of two ice skaters on an ice skating rink. So if you and your friend go to the ice skating rink and you stand in the middle and you then push off of each other, both of you are going to go in different directions.

Now, if you go and stand with a big group of people, so there's like 10 of your friends and family and just you, and then you push off of them, what's going to happen is you're actually going to go farther away than the group of 10 people. They might move just a little bit, but they're not going to go as far apart as if it were just you and your friend.

The reason why it's easier for you and your friend to push each other away and why when you push off your friend, you guys go in equal and opposite directions and why that's different from when you push off your group of friends and family is because of something we call the moment of inertia. Things like to stay put and the bigger they are, the harder they are to get moving.

And so because you have your big group of friends and family, and those are bigger than just one person, it's going to be harder to get them moving than it is someone who is a single person. And the simpler thing would happen if everyone,

on Earth, all 8 billion people went to the same exact spot and said, "One, two, three, jump," what would happen is the Earth and all of the people would move in the opposite direction. So the Earth would indeed move. But because the Earth is so, so much bigger than everyone else combined, you wouldn't even really notice it. I don't even know if scientists could even measure that precisely.

I gotta say, that's kind of comforting to know. So the heavier something is, the harder it is to get it moving by pushing or pulling, or to stop it if it's already moving. Think about a tennis ball rolling down a hill. It's small and light, and you could stop it with your hands. But if it were a big heavy boulder rolling down the hill towards you, you're gonna want to get out of the way.

The same thing is true about the Earth. If all the people on the planet went to one spot and jumped, there would be more than 8 billion bodies pushing down against the Earth. But the Earth is so big and heavy that even the weight of all our bodies combined would barely even jostle it. Well, that takes care of that what-if question. So what now? ♪

If you like this episode, take a second to subscribe to Moment of Um wherever you listen to podcasts. And if you love brain benders, check out the Brains On podcast where we have a whole episode all about other what if questions like...

What would happen if a fly flew through the window of a bus? Would it have to keep moving to avoid hitting the back window? Well, you'll have to listen in to learn more. Want to see our shows come to life? Head to YouTube where we've got awesome animated Brains On episodes. Just search Brains On Universe on YouTube and subscribe. If you have a question, we'd love to help you answer it. Drop us a line by going to brainson.org slash contact. See you next time.

and the next day and every weekday. Until then, um... I guess it really is like Van Halen said. Might as well jump.