1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does air keep weight

  1. May 25, 2009 #1
    So I am talking about any gas. I anm going to doubt that the air really has some weight. What is the simplest expeiment that could prove that the air does have weight? May be baloons on a balance, when one of them is blown up, the weight on that pan decreases. I suppose that the gas molcules have no weight. The air has weight only due to the dust paricles and the water droplets/? solved in it which give the impession that the air has weight.

    What expeimental setup would give purely the weight of the gas molecules excluding any of the entities solved in air?
     
  2. jcsd
  3. May 25, 2009 #2

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Take a metal can with an airtight lid.
    Fill with boiling water, screw on lid, watch can collapse as steam inside condenses.
     
  4. May 25, 2009 #3
    Weight of air on earth

    Exactally how much weight is felt per unit suare of area on earth? Is it same on every direction or is more in vertical direction?
     
  5. May 25, 2009 #4

    russ_watters

    User Avatar

    Staff: Mentor

    Re: Weight of air on earth

    Air pressure at sea level is approximately 14.7 psi and yes, air pressure acts the same in all directions from a point.
     
  6. May 25, 2009 #5

    russ_watters

    User Avatar

    Staff: Mentor

    A similar experiment with a plastic bottle partially full of water and heated in a microwave would work too.
     
  7. May 25, 2009 #6
    Air (80% nitrogen, 20% oxygen) has a mass of about 1.29 grams per liter at sea level and 20 degrees C. Water is about 1000 grams per liter. One way to weigh air is to get a 1 liter (or larger) lightweight metal tank, pump all the air out, weigh it accurately, let the air back in, and weigh it again. You should see a 1.2 or 1.3 gram difference.
     
  8. May 25, 2009 #7
    Re: Weight of air on earth

    What's the source of this pressure then?
     
  9. May 25, 2009 #8
    Bob_s,
    Air isn't only 80% nitrogen and 20% oxygen in my view. It contains so many other suspensions. I was talking about the pure pure air, with no other particles mixed in.

    mgb and russ_waters,
    I think the experiment shows the pressure of air when the can inside is free of air. But it wouldn't tell the air weight quanitatively, isn't it so? Am I confusing the air pressure with the air weight?
     
  10. May 25, 2009 #9
    Re: Weight of air on earth

    If the pressure is same in all directions and is not reinforced in the vertical direction by the gravity, is it sound to say that air does not have some particular weight? Or it is not affected by the grvity.
     
  11. May 25, 2009 #10

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Re: Weight of air on earth

    Air is a fluid so the weight pushes down but it can flow around corners and so the force is felt the same everywhere.
    Imagine a large box full of metal ball bearing with a hole in the side, even though the weight is only downwards there would still be a force pushing the balls out of the side.
     
  12. May 25, 2009 #11

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    It shows you that air has weight.
    to measure the actual weight you need more sensitive equipement, air weighs around 1.2Kg/m^3 so to get a reasonable difference in weight you need a large volume container that doesn't collapse when you pump the air out of it.

    Alternatively you can rent a cylinder of compressed air and weigh it full and empty.
     
    Last edited: May 25, 2009
  13. May 25, 2009 #12

    russ_watters

    User Avatar

    Staff: Mentor

    Air pressure, 14 psi, can be thought of as the weight of a 1 square inch column of air.
     
  14. May 25, 2009 #13

    russ_watters

    User Avatar

    Staff: Mentor

    Re: Weight of air on earth

    It is the weight of the air.
     
  15. May 25, 2009 #14

    russ_watters

    User Avatar

    Staff: Mentor

    Identical threads merged...
     
  16. May 25, 2009 #15
    Re: Weight of air on earth

    Thank you for the Ball bearing example. It helps understand why air should have pressure in all directions and not solely in the downward direction. As now I can understand there would be more pressure on the ball bearings to flow out of the hole if the bearings height were more. It definitely explains for the horizontal force.

    Howevere if we consider the case when one is inside a room with 7 feet height. The room may be close except very small holes, with windows and doors closed. So would there be some difference in the pressure or weight due to the air? Apparently now there is not much weight on a person. Rest of the air weight is being supported by the roof. Pressure should reduce.

    Your example once seemed to solve my problem but it has caused some more confusing now.
     
  17. May 25, 2009 #16

    russ_watters

    User Avatar

    Staff: Mentor

    No, the air flows into the holes and fills the room, equalizing the pressure. A house's roof could not possibly support the weight of the air above it. A 10'x10' section of roof would have to support more than 200,000 pounds!
     
  18. May 25, 2009 #17

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Compared to the weight of air ( 10tons/m^2) the walls act like elastic balloons.
    The weight of the air outside pushes in on the walls, roof and windows this presses on the air inside and means you have the same pressure on both sides.
     
  19. May 26, 2009 #18

    rcgldr

    User Avatar
    Homework Helper

    Go to a scuba shop. Weigh a tank, then let them add 80 cubic feet of sea level pressure air into the tank. Weigh the tank again and it's is now 6 pounds heavier because of the 80 cubic feet of air forced into the tank.
     
  20. May 26, 2009 #19
    Dear mgb_phys,
    Sorry I can't digest the idea that roof and walls act like elastic baloons. Weight as per unit square? means pressure? Hope it doesn't say in m^3


    Jeff Reid,
    I am so sorry that I have to repeat again. The air I would weigh would alsocontain water vapours, dust,carbon particles or many other suspensions. My question have been about the exactly pure air, just gaseous molecules, no other contaminations. Hope you understand my point.
     
  21. May 26, 2009 #20

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    mabs239 these are very good questions.
    First the weight of air at sea level (ie the air pressure ) is 10tons/square metre.
    It isn't so much that air is heavy (it only weighs 1/800 as much as water) but there is about 10,000 m of it above your head.
    The density of air (how much a box of air would weigh) is only about 1.2Kg/m^3 but if you stack 10,000of these boxes on top of each other you get atmospheric pressure.

    So if you use bathroom scales why doesn't the air press down on the top surface of the scales (roughly 0.1m^2 area) to make you weigh 1ton more?
    This is because air is a fluid, it pushes down inside/underneath the scales and pushes the top of the scale up with equal force. (remember the ball bearings)
    If you took some scales and pumped all the air out of the inside then the top would be pushed down by an extra ton of force. And they would be crushed - remember a vacuum doesn't suck things in - it's the weight of the air outside pushing them down.

    So next obvious question is why is there air pressure inside a room? If there is only 1m of air above my head rather than 10,000m why is there the same air pressure?

    Again back to the ball bearings! The air is pressing down on the roof (and walls and windows) with 10tons/m^2, unless you live in a nuclear bunker the walls and roof have no strength on this scale and so press down on the air inside which pushes back until it is the same pressure as outside.
    It's a bit like a bicycle pump - if you push down on the pump with your arm (pretend that is the outside air pressure) then the piston (the roof) moves down until the force of the air inside the pump is pushing back - this happens when the pressure inside the pump is the same as the pressure you push down with.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Does air keep weight
  1. Does hot air rise? (Replies: 12)

Loading...