Ask HN: Vacuum is so powerful, so why it doesn't suck the Earth's atmosphere?

4 points by Houssameddine 2 years ago | 5 comments

I read about the experiment of Magdeburg hemispheres [1], where they attached two halves of a sphere only by emptying the sphere from air, and they couldn't pull them apart using 16 horses!

So why doesn't the vacuum in outer space absorb the Earth's atmosphere if it's that powerful?

The obvious answer would be gravity, but how? I mean, gravity isn't as strong as 16 horses, right?

[1] https://en.wikipedia.org/wiki/Magdeburg_hemispheres

compressedgas 2 years ago |

Space isn't a vacuum. It is filled with ultra low pressure hydrogen.

If you did that vacuum sphere experiment in space, it wouldn't take 16 horses to pull it apart. The force that held the halves of the sphere together was the weight of the displaced atmosphere.

Consider the state of the sphere halves before the inside was emptied. The pressure on the inside and outside was about 14 pounds per square inch. After it is only 14 PSI on the outside and 0 PSI on the inside thus the two halves of the sphere are being held together by that 14 PSI of atmospheric pressure. If you were to take this sphere into space. The pressure holding it together would reduce and it would easily separate.

Gravity is what holds an atmosphere to a planet. But it was the weight of that atmosphere which held the sphere halves together.

RetroTechie 2 years ago | |

> The force that held the halves of the sphere together was the weight of the displaced atmosphere.

More precisely: the force applied by the weight of the column of air above the semi-spheres.

Weight of the column of air -> air pressure -> force on semi-spheres.

As parent noted, high up in the atmosphere that is a small force. At sea level, ~1 kg/cm^2.

So: yes, it's gravity.

thesuperbigfrog 2 years ago |

>> I mean, gravity isn't as strong as 16 horses, right?

It is stronger due to the mass of the earth which is about 5.9722×10^24 kg.

If the opposite were true, then the earth would not have an atmosphere.

smt88 2 years ago |

Google is your friend: https://scijinks.gov/pressure/#:~:text=word%20is...-,Gravity....

al2o3cr 2 years ago |

Vacuum is LITERALLY NOTHING. It doesn't have "power".

The Magdeburg hemispheres are held together by the weight of the atmosphere pressing inward. That is ultimately about gravity.