11 ms·
Yes. You can think of it as: If you’re surrounded by an equal amount of mass in every direction (because you’re at the center of a set of spherical mass shells)
by drbaba 3y ago
Yes. You can think of it as: If you’re surrounded by an equal amount of mass in every direction (because you’re at the center of a set of spherical mass shells), then the gravitational force in every direction will cancel out.
- pbhjpbhj 3y agoAgreed. >If you’re surrounded by an equal amount of mass in every direction Which is a reasonable approximation for being 'out in space', but perhaps not an entirely intuitive one.
- Qwertious 3y agoSo if a cup of water were in the centre of the earth, and every layer of the earth above the centre wasn't there to compress it, then it would explode out! Seems perfectly reasonable, if a bit useless.
- nimchimpsky 3y agono it would be weightless, as all the gravity would cancel each other out.
- pja 3y agoNo, it won't explode. If you somehow made a spherical hole in the middle of the earth and placed a cup of water inside the hollow (anywhere inside, doesn't have to be the centre) it will just sit there: Newtonian gravity is 0 inside a uniform spherical shell of mass. (Einsteinian gravity is probably mostly 0 but you'll get frame dragging effects if the shell is rotating I would imagine.) You can do the maths to prove this yourself if you want.
- misja111 3y agoYes but I think OP was referring to the lack of pressure. Without gravity, no pressure -> water will violently boil.
- pja 3y agoFill the hole with air at 1atm & it’ll sit there very slowly evaporating though, same as it does on the ISS or on the surface of the earth. Gravity is irrelevant.
- misja111 3y agoYou don't seem to understand where that 1atm air pressure on the surface of the earth is coming from. Hint: on the surface of the Moon there is none. In the ISS the air pressure is artificially kept at a certain level. To make a proper comparison, you should check what water would do outside of the ISS.
- pja 3y agoNo, I understand precisely. The context of my original comment was someone expecting that a cup of water at the centre of the earth would explode due directly to gravitational effects. This is wrong, as I pointed out. Even on the surface of the earth it is the air pressure that prevents the water boiling off instantly, not gravity itself. On the surface of the earth that air pressure is due to the effect of gravity on the atmosphere, but you can supply that air pressure by a number of other means in other places. The air pressure inside the ISS is not there due to gravity after all! I suggest to you that if we can posit a hole at the centre of the earth, then we can fill it with air at 1atm if we want to - we are in the realm of mathematical models here, not reality after all :)
- yorwba 3y agoIf a cup of water at room temperature were on the surface of the earth and every layer of the earth above it wasn't there to compress it (so no atmospheric pressure) the water would violently boil immediately. If the cup is very tall or you're talking about a cup of very cold ice, the water's own gravitational pressure could keep the bottom stable, but to prevent the top from boiling/sublimating off, you'll need a lid to keep it pressurized. Moving the experiment to a zero-gravity environment doesn't change the pressure considerations much.
- sliken 3y agoIndeed, and as a related situation. You could be inside a black holes event horizon and experience zero gravity/acceleration. You just need two blackholes to cancel out the gravity.
- Ntrails 3y agoI feel like being in the middle of two black holes would be more like being ripped apart than zero gravity/accel.
- sliken 3y agoDepends on the size of the black holes, spaghettification because of a high gravity gradients is a problem when you are close to the center of the black hole. Larger black holes make the much less of a problem, so you can cross the event horizon without any problems.