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I Is atmospheric pressure gravitational or kinetic?

  1. Dec 6, 2018 #1
    I keep seeing a popular question asking about atmospheric pressure "crushing" us. The word "crush" throws me off.

    It is my understanding that air molecules create pressure due to collisions. In other words, the molecules exert pressure due to having kinetic energy - and the more molecules you have, the more collisions you have.

    Under that mechanism, wouldn't it be technically incorrect to use the term "crushing" to describe atmospheric pressure?

    The way I see it, gravity causes air molecules to increasingly weigh down on the next level of air below it, continuing all the way down. This causes there to be greater density at the bottom, which in turn means more collision.

    So is atmospheric pressure due to gravitational pull? Or is it due to particle collisions (in this case gravity's role being to only "densify" these particles)? Or both?

    When people speak of crushing, does the atmosphere actually crush us in the gravitational sense of pulling down or weighing down molecules against us? Or does the weighing down of molecules only create a density gradient, and from there "kinetic energy" of the density field "crushes" us? Or perhaps a contribution of both?
     
  2. jcsd
  3. Dec 6, 2018 #2

    jedishrfu

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  4. Dec 6, 2018 #3
    I think I get it now. So basically, everything on top which is weighing down on the bottom molecules is causing compression of the bottom molecules, which in return exerts an equal but opposite force in the form of kinetic collisions. Their compression has reached a point where they can push back with equal force against everything on top. In other words, the bottom is just more compressed gas, and the molecules exert their collisional force while being more closely packed. Therefore their kinetic force/area ratio is increased.
     
  5. Dec 6, 2018 #4

    jedishrfu

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    Yes, I think that's right.

    Here's Veritasium on it:

     
  6. Dec 6, 2018 #5

    russ_watters

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    Jedi's correct, but to answer your questions a little more directly:
    That's how gases exert pressure. They *have* pressure due to how they are constrained (in the case of our atmosphere, by gravity). Contrast that with a pile of books or jar of marbles, which exerts pressure just by sitting there.

    [edit: it's momentum not kinetic energy, but that isn't key to this discussion.]
    I don't see a semantic difference. To me, "crushing" is what happens to the object, not what is happening to the air, water, hydraulic press, etc.
     
  7. Dec 6, 2018 #6
    I had to think about this for a minute. It makes perfect sense now. You are da man.
     
  8. Dec 6, 2018 #7

    hilbert2

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    If the gravitational pull of the Earth were to suddenly become a lot weaker, the air molecules would fly away because of that kinetic energy they have. It takes a certain temperature to have a given amount of kinetic energy per gas molecule, and a certain confining force (gravity) to keep the gas molecules concentrated near the Earth's surface.
     
  9. Dec 6, 2018 #8
    I guess what made it click for me was going through a thought experiment in my head. You lay down one layer of air molecules around the surface of the earth. Next you lay down the second layer. This second layer will weigh less. The first layer will now be weighted down onto the 1st layer. There is space between them because the molecules resist being confined (due to kinetic energy). Next add a third layer, having even less weight, but still weight nonetheless. Now the 1st layer will experience the weight of layer 2 and 3 weighing down on it. The space between the 1st and second will become smaller than the space between the 2nd and 3rd. Now add a 4th layer. The space between 1st and 2nd layer will shrink even further (i.e. be under even more pressure). The 2nd layer - 3rd layer space will shrink as well, but it will be it's first shrinkage. Add a 5th layer. Now the space between 1st and 2nd has shrunk 3 times. Space between 2nd and 3rd has shrunk 2 times. And the space between 3rd and 4th shrunk 1 time. Wash rinse and repeat. You end up with a more dense/compressed (smaller space/area) gas atmosphere at the surface of earth, that is compressed to have an equal but opposite force to everything else weighing down on it. The repulsion of bouncing molecules is what creates the force that exerts back with equal force on the upper column of molecules being weighed down.
     
  10. Dec 6, 2018 #9
    Gotcha. It's that confinement that creates a large force/area. I wonder theoretically what the kinetic energy without the gravity would be like. Like if you had a container around earth, keeping the molecules bouncing around inside earth. Minus the gravity. Probably just way less pressure? Because shrinking that container towards the earth surface will reduce space, and therefore increase collisions per smaller area. It all makes conceptual sense now.

    Thank you guys! I have trouble taking things for face value. I need it broken down (WAY down). Lol
     
  11. Dec 6, 2018 #10

    That video was hilarious and entertaining.
     
  12. Dec 6, 2018 #11

    hilbert2

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    In a significantly lower air pressure, there wouldn't be liquid water on Earth. That would decrease the heat capacity of Earth's surface dramatically and the day and night temperatures could differ by as much as 100 Kelvins, causing an equally large variation in the gas molecule KE:s
     
  13. Dec 6, 2018 #12
    I have to thank you for this bit one more time. It was KEY.
     
  14. Dec 6, 2018 #13

    russ_watters

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    You're welcome!
     
  15. Dec 11, 2018 at 3:46 PM #14
    The UCAR reference is clearly designed for children. If air pressure represents the weight of the overlying air, how do you explain the Bernoulli Effect. Air pressure demonstrably responds to wind direction and wind velocity. Anybody who has observed a barometer "pumping" during a windstorm can testify to this. Neither wind direction nor wind velocity have anything to do with the "weight" of the overlying air.

    Take a tall building exposed to a strong wind with rooms on the fortieth floor with open windows on all four sides. Each of the four rooms contains a barometer and an observer. At the same time, we have a nearby balloon drifting with the wind and an observer and a barometer in the basket at the level of the fortieth floor. Each of the five barometers will observe a different atmospheric pressure (the two rooms normal to the wind flow will have very similar measurements). Are we to assume that this difference is due to the different masses of overlying air? I don't think so.

    The "weight/force" fallacy dates back to the days when atmospheric molecules were thought to be relatively stationary and possessed of infinitely-expandable "spheres of influence" that were capable of transmitting weight-forces by molecule-to-molecule contact. Bernoulli put an end to that nonsense in 1738, but the concept unfortunately lives on in popular writings. Molecule-to-molecule impulse transfer is a function of air temperature, not in any way related to air pressure.

    Finally, I should like to point out that the barometric formula assumes conditions of equilibrium, and the free global atmosphere has never been observed in a condition of equilibrium. Moreover, that same formula contains a whole slew of ridiculous assumptions that cannot rationally be applied to the free atmosphere, including one (uniform columnar cross-sectional area) that only active members of the Flat Earth Society can accept.
     
  16. Dec 11, 2018 at 3:51 PM #15

    russ_watters

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    Bernoulli's equation includes and simplifies to the hydrostatic pressure equation when there is no motion. The Bernoulli effect generally refers to air in motion and is different from what causes static air pressure. On most days, the effects of weather only affect a <1% change from the hydrostatic air pressure.

    Please don't overcomplicate/add extra things that don't apply.
     
    Last edited: Dec 11, 2018 at 4:20 PM
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