Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Why do gases have weight?

  1. May 19, 2016 #1
    In studying physics over many years, I am still perplexed that gases have weight. I understand that gases are composed of molecules, but if they are moving in all directions with much space between each molecule, how is there a downward weight that can be measured with all that space in between? Is this somehow due to intermolecular forces? How does a weight force get transferred though empty space between molecules? Thank you?
     
  2. jcsd
  3. May 19, 2016 #2
    Gases are made of atoms, atoms have mass.
    Anything with mass is subject to gravity.
     
  4. May 19, 2016 #3
    (Note: I assume you mean between the molecules of the gas and the molecules of Earth). Old basic Newtonian answer: all massive objects attract eachother. How? Newton did not know how exactly; he thought there was some reason for it, but he did not know what it was (EDIT: you can see what Newton said here in this source). Modern Einsteinian answer: General relativity; i.e. "spacetime tell the masses how to move".
     
    Last edited: May 19, 2016
  5. May 19, 2016 #4
    Weight is a measurement of mass under gravity. Gravity distorts space. The individual atoms and molecules can move about in all the different ways that they do, but they always move through space which is bent by gravity.

    There is an anology of two people playing table tennis on a train. No matter where the ball (or atom) goes or what it does inside the train (Earth), it always goes where the train takes it.
     
  6. May 19, 2016 #5

    russ_watters

    User Avatar

    Staff: Mentor

    They bump into stuff.
     
  7. May 19, 2016 #6

    Drakkith

    User Avatar

    Staff: Mentor

    The atoms and molecules exert a force on an object because they collide with that object. For example, in a pressurized air tank a large number of molecules slam into the walls of the container every second, which is where the pressure comes from (pressure is force per unit area). That's why the pressure inside the container increases if you heat it up. And why fires are so dangerous when you have pressurized containers of gas around. The increased temperature leads to an increase in the average velocity of the gas molecules, which increases the rate at which they slam into the container walls and the force per collision.

    The weight of the gas is transferred through collisions, just like pressure is.
     
  8. May 20, 2016 #7

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Gravity acts on gas atoms/molecules the same as it does any other atom/molecule.

    Gas atoms/molecules bounce off each other but if you look carefully at a box of gas the density is greater at the bottom than the top. This is due to gravity. The difference is more noticeable if the box is very large - say the size of the atmosphere.
     
  9. May 20, 2016 #8
    An interesting question. The gas is composed of molecules with masses mi, so one might assume the weight would be given by ∑mig. But we measure the weight of an object by its force on another object such as a scale. As most of the molecules are flying around inside the container as FEqMA says, they cannot contribute to a measurement of the weight of the gas.

    According to the responses here, the weight of the gas varies with temperature.
     
  10. May 20, 2016 #9

    russ_watters

    User Avatar

    Staff: Mentor

    Neither of those statements are true and they aren't implied by the explanations given. The pressure of the atmosphere is caused by its weight and is transmitted all the way from space to the ground by molecules bumping into each other.
     
  11. May 20, 2016 #10

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    If you pick up a box of tennis balls and stand on some scales the scales will indicate your combined weight. If you shake the box it won't weigh less (on average).
     
  12. May 20, 2016 #11
    And if the temperature is higher in a closed system, don't they bump into each other with more force?
     
  13. May 20, 2016 #12
    Yes, and more force translates as more pressure overall within the closed system.
    Individual molecules mover faster but not in any preferred direction, so for the overall system the increased momentum of individual molecules cancels out
    This however has no effect at all on the weight of the contents of the system.
    All the molecules still have the same rest mass and are still subject to gravity regardless of what temperature they are at.
     
  14. May 20, 2016 #13
    I won't challenge the accuracy of your statement, but let's follow up just for understanding and to thoroughly address the OP's question. When you stand on the scale initially, all the weight of the balls ∑mig affects the scale. Now suppose the balls each have some random motion. Do you agree that any ball while suspended in air cannot contribute to the weight as measured by the scale? As for the ones impinging on the box, they would do this equally at the top and bottom on average. So this indicates the weight would be less on average when they are moving.
     
  15. May 20, 2016 #14

    Drakkith

    User Avatar

    Staff: Mentor

    I believe the ones colliding with the top of the box do so with less average velocity, as they are slowed by gravity as they move upwards. Conversely, the ones colliding with the bottom will have more average velocity since gravity accelerates them as they fall.
     
  16. May 20, 2016 #15
    The weight of a liter of gas at atmospheric pressure will indeed be lower at higher temperatures. But A closed liter of gas would contain the same mass, and would weigh the same at any temperature. It may not be obvious, but the atmosphere is "closed".

    Every molecule of gas is moving, but also is being accelerated by gravity towards the earth center of mass, the bottom of the container. So the average velocity of the molecules striking the bottom, is slightly greater than the average velocity of the molecules striking the top. Assuming they elastically recoil, they transmit more force down than up.

    All the mass will always be feeling the force of gravity, downward, just as you do standing on a scale. You are supported by your feet, and your legs, and a column of body. The air is supported by a no less real column of air molecules, just spread so much more thinly than your solid body. And the force downward still registers, even though the gas molecules are so scattered. Every molecule feels that force, and there is a net acceleration of molecules downward. They strike slightly harder in the down direction in aggregate.
     
  17. May 20, 2016 #16

    russ_watters

    User Avatar

    Staff: Mentor

    Yes. But that's a totally different issue from what we were discussing. And even then, for a closed system (not the atmosphere), the two statements I quoted are still wrong.
    The motion of the balls is not random (due to gravity pulling them down), but yes, only balls that impact the surface impart a force on it.
    In orbit, with no gravity, that would be true. But becuase of gravity, the force applied to the bottom is greater than the force applied to the top. Why? Because the ones moving down accelerate before hitting the bottom and the ones hitting the top decelerate before hitting the top.
     
  18. May 21, 2016 #17
    FEqMA: Applied to gases, this appears to be the key to answering your question.
     
  19. May 21, 2016 #18

    FactChecker

    User Avatar
    Science Advisor
    Gold Member

    I think it would be more clear to say they are the only ones to directly impart a force on the surface. The balls in the air are there because another ball bumped them upwards. That gave that other ball a downward velocity that made it hit the surface.

    I think it is better to look at the OP in a different way: Clearly every molecule of gas would want to settle down on a hard (impermeable) surface because of its weight and form a layer of gas molecules on the ground. Their weight would apply a force to the ground. So the OP question should be why that doesn't happen. The answer is that the gas molecules bump randomly into each other and bump some upward. Those that are bumped upwards, in turn, bump others up even higher. etc., etc., etc.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Why do gases have weight?
  1. Why do we have mass? (Replies: 13)

Loading...