Center of mass (COM) it’s easy balancing act or a trick to try at home! All you need is 2 forks, a quarter, a cup, and some patience.
Why does this look so weird? It all has to do with center of mass. COM is hard to define, but almost everyone has a great intuition for it! You find the COM of objects when you’re balancing things. Once an object is balanced, wherever you are holding up that object is where it’s COM is. You could probably guess where the center of mass is for a lot of symmetric objects. For example, a ruler’s COM is in the middle. But if you add some extra weight to the end, it’s COM will shift!
A fork is pretty asymmetic, and that shows in it’s COM.
If you try to balance 2 forks on your fingers, you probably won’t win. This is where things get weird– the center of mass does not necessarily need to be within the object! For 2 forks, it ends up being just right outside of it.
Which makes the “trick” work. If you stick something, like a quarter, between the 2, now the center of mass of this collection of objects is now on the quarter. You will intuitively find the exact place on the quarter where the center of mass lies, when you achieve balance on something like the edge of a cup (pro tip: the more rigid the edge of the glass is, the better. We used the bottom of the cup here because that was less rounded, and hence stuck better, than the top of the cup). There you have it! Rather than thinking of it as a trick, think of it as you showing a weird property of physics in a simple way!
You’ve probably heard of liquid nitrogen ice cream before. It’s made by mixing together ice cream ingredients with liquid nitrogen, which turns into a gas at -321º F. Learn more about that at https://www.thoughtco.com/cryogenics-definition-4142815. The intense coldness is what turns the ice cream ingredients from a liquid to a tasty solid.
So by this logic, you should be able to make ice cream by just adding something really cold to your ice cream ingredients. Solid carbon dioxide, AKA dry ice, is a good candidate to experiment with. To be a solid, carbon dioxide has to be at least -109º F! When you mix dry ice into ice cream ingredients, something interesting happens. It does cool it down, but you additionally get a lot of bubbles. As the carbon dioxide sublimates from solid to gas, little pockets of gas gets trapped underneath the ice cream, and they escape to the surface in little bubbles of CO2.
Why don’t these bubbles form with liquid nitrogen? Both dry ice and liquid nitrogen are turning into gases. But, it’s a lot easier for a solid to sink to the bottom of the ice cream mix than a liquid. When making liquid nitrogen ice cream, if you leave the nitrogen alone, you won’t see it sink down to the bottom of the ice cream. So when the liquid nitrogen evaporates, the gas simply rises from the top of the mixture into the room.
The little carbon dioxide bubbles that escape are the same as bubbles in a soda. That’s why it’s called carbonation! So in a way, you can create a sort of carbonated soft serve by mixing ice cream ingredients with dry ice– but be ready for it to be a bubbly mess.
Note: You should never ingest liquid nitrogen or dry ice. It will burn you and harm you. When people eat liquid nitrogen ice cream, they’re just eating the ice cream ingredients, with all the liquid nitrogen changing phase into a gas before consuming it. The liquid nitrogen merely acts as a mechanism to cool down the ingredients.
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