1. I have learnt that due to upward thrust of air, the weight of an object is reduced to some extent than the real weight. What about of the effect of air pressure or atmospheric pressure on weight? Should it not result in increase in weight?
  2. jcsd
  3. mfb

    Staff: Mentor

    "upward thrust"?

    If an object moves upwards, air can move downwards - the change in potential energy in the system is less than the change in a vacuum. This looks like a reduced weight.
  4. russ_watters

    Staff: Mentor

    I think that was buoyancy.

    In order for the pressure itself to have an effect, it would have to be constrained to push down on the scale but not up. Balances don't have vacuums underneath them.
  5. sophiecentaur

    sophiecentaur 14,717
    Science Advisor
    Gold Member

    The air surrounding an object is pressing in all around it. The pressure underneath (up) is slightly more than the pressure at the top (down) so this results in a net upthrust. It's just the same as for an object in water but the effect is much more noticeable.

    If you try to lift an object with a very smooth bottom from a smooth surface, air can't get in underneath and you get the full effect of atmospheric pressure from above but none from below. Until the air leaks in underneath, you need a massive effort to lift it.
  6. yes...because water is so much more dense than air.

    Also, as a note, there is no 'real weight' of an object.....it's weight depends on where you measure it....weight in water is different from weight in air which is different from planet to plant, for example.

    Mass is constant...Weight = mass x gravity which is a form of F = ma.
  7. of course the upward thrust of air reduces the weight. But what about the air pressure above us? Does it not increase the weight?
  8. There is pressure above you pushing down and trying to increase your weight, but there is just as much pressure below you pushing up trying to decrease your weight. So they ALMOST balance out. But, there's actually slightly more air pressure below you than above you, so the overall total effect is that your weight actually gets decreased. This is the "upward thrust" you speak of, otherwise known as buoyancy. It takes the pressure above you into account.
  9. sophiecentaur

    sophiecentaur 14,717
    Science Advisor
    Gold Member

    "Of course" there is an effect of pressure in both directions. However, because pressure increases with depth, there is more upward force than downward force. Hence, upthrust.

    [Edit: snap!]
  10. haruspex

    haruspex 16,061
    Science Advisor
    Homework Helper
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    2014 Award

    When ambient air pressure is higher (for a given temperature), the density of the air will be higher, so buoyancy will be increased - just slightly.
  11. Thanks , for appropriate explanation
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