An flying object needs lift,but this lift is like buoyant force in air?

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An flying object needs lift,but this lift is like buoyant force in air??

just like when any object is immersed in water it feels lighter because a net upward force is acting on it same way do net upward force is acted by air when any object is flying in air?? and that net upward force is more than gravitational force?:cry:
 

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  • #2
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There is an upward force(upthrust) on an object in the air but this is approximately 800 times smaller than the upthrust there would be on the same object in water.The difference is due to the density of air being approximately 800 times smaller than the density of water.For most objects the upthrust in air is so small that it can be ignored but not so for low density objects like hot air balloons which can float because the upthrust can be bigger than the weight.
 
  • #3
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i am also not getting this but i think so probably [email protected] i think soo if upward force is more in water than air,then an obect should float in water also...
 
  • #4
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Yes. An object that floats in air will also definitely float in water.
However, airplanes do more than just "float". "Lift" is something separate from the buoyant force.

This is what really keeps planes in the air: http://en.wikipedia.org/wiki/Bernoulli's_principle
 
  • #5
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@archosaur i think so cjs9 wants to ask that because of an upward pressure of air aeroplane or any other object flys in air?? and i think soo yesss.he had jst compared with buoyant force.
 
  • #6
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Yes. An object that floats in air will also definitely float in water.
However, airplanes do more than just "float". "Lift" is something separate from the buoyant force.

This is what really keeps planes in the air: http://en.wikipedia.org/wiki/Bernoulli's_principle

I would think that the density of air molecules decrease as the flow over the long top of the wing and the relative vacuum of that air sucks the wing upward. But the "sucking" force of a vacuum is not actually sucking but pressure from the higher-density medium surrounding the vacuum. So when a hot-air balloon expands because its hot air is less dense, which causes it to seek entropy with surrounding air, isn't this similar to the lower pressure air above the wing seeking to equalize its pressure with adjacent air above it? Granted it's not the same as buoyancy but both involve a tendency to converge to pressure-equilibrium, right?
 
  • #7
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Bernoulli's principle isn't the only factor, angle of attack is important too. (Plenty of threads here on this subject.)

Brainstorm, a hot air balloon expands because of the higher temperature of the air inside produces a greater pressure than the outside air. Density is irrelevant. I can get a balloon to expand with a substance more dense than air (think water balloon) or less dense than air (helium balloon) purely because the internal pressure is greater than the external.

An aircraft's overall density is significantly denser than the surrounding air. Buoyancy plays absolutely no part in it's ability to fly. It's all about lift production through bernoulli's principle and angle of attack.
 
  • #8
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(sigh) it's so annoying to correct strawmen.
Brainstorm, a hot air balloon expands because of the higher temperature of the air inside produces a greater pressure than the outside air. Density is irrelevant. I can get a balloon to expand with a substance more dense than air (think water balloon) or less dense than air (helium balloon) purely because the internal pressure is greater than the external.
I meant density of atmospheric air, which is the same thing as pressure. I think I said that in my post.

An aircraft's overall density is significantly denser than the surrounding air. Buoyancy plays absolutely no part in it's ability to fly. It's all about lift production through bernoulli's principle and angle of attack.
I didn't say buoyancy was the relevant factor. I said that pressure causes balloons to expand and also causes wings to lift. I said that in both cases, denser air seeks equilibrium with less dense air, which causes the less dense air to move in the direction of the denser air. Actually, this is the same effect that causes buoyancy, isn't it? Gravity compresses molecules and the compressed molecules push the less dense object away from the higher pressure area, no? Only in the case of the wing, I think the pressure differential is greater than is possible with static air because the air moving over the wing increases the psi-differential between the top and bottom of the wing. Now, I'm getting confused so I hope I haven't lapsed into obfuscation of reason.
 
  • #9
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I meant density of atmospheric air, which is the same thing as pressure. I think I said that in my post.

No, you didn't. You said:
So when a hot-air balloon expands because its hot air is less dense,

Density is not the same as pressure, atmospheric or otherwise.
I didn't say buoyancy was the relevant factor.

I didn't say you did, this part wasn't aimed at you.
I said that pressure causes balloons to expand and also causes wings to lift.

As above, you said density causes the balloon to expand and this is simply untrue.
I said that in both cases, denser air seeks equilibrium with less dense air, which causes the less dense air to move in the direction of the denser air. Actually, this is the same effect that causes buoyancy, isn't it? Gravity compresses molecules and the compressed molecules push the less dense object away from the higher pressure area, no? Only in the case of the wing, I think the pressure differential is greater than is possible with static air because the air moving over the wing increases the psi-differential between the top and bottom of the wing. Now, I'm getting confused so I hope I haven't lapsed into obfuscation of reason.

Less dense fluids will float on denser ones. You can only have buoyancy within a gravity field. Dense fluids will 'push' less dense ones away from gravity source - in this case upwards. That's how a balloon flies. An aircraft uses a completely different principle involving pressure and angle of attack.

Again, pressure has nothing to do with density. The pressure inside the hot air balloon and a water balloon is greater than atmospheric so it inflates. However, the density of a water balloon is greater than atmospheric and the density of the hot air balloon is less than atmospheric.

A hot air balloon is a lighter than air vehicle. An aircraft is a heavier than air vehicle. They use different principles to attain flight.

EDIT: Density = Mass/Unit Volume, Pressure = Force/Unit Area. Two completely different things.
 
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  • #10
rcgldr
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Why are there replies when the OP was either banned or quit after making just one post here? That person won't be able to read the replies. I recall a similar thread that was deleted 1 or 2 days ago.
 
  • #11
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Density is not the same as pressure, atmospheric or otherwise.
In a uniform gas mixture, heat-energy gets expressed as momentum of the constituent molecules. The higher momentum of the molecules forces them away from each other and they maintain a greater average distance. This results in lower density as well as greater pressure than cooler gas of the same composition, no?

As above, you said density causes the balloon to expand and this is simply untrue.
Ok, you're right. It is the pressure and the lower density is also a result of the pressure. I mixed up causality.

Less dense fluids will float on denser ones. You can only have buoyancy within a gravity field. Dense fluids will 'push' less dense ones away from gravity source - in this case upwards. That's how a balloon flies. An aircraft uses a completely different principle involving pressure and angle of attack.
But couldn't you say that the longer distance the molecules have to travel over the wing spreads them out in a way that lowers their density? Doesn't this produce a relative vacuum that moves toward denser air above it to seek equilibrium pressure? I.e. doesn't the air around the wing basically attempt to harmonize its pressure/density around the entirety of the wing (top and bottom) and by doing so cause the whole wing to move toward the relative vacuum being generated on the top?

A hot air balloon is a lighter than air vehicle. An aircraft is a heavier than air vehicle. They use different principles to attain flight.
Lighter/heavier than the volume of air displaced at a given moment, maybe, but doesn't the plane have to displace a sufficient amount of air in a certain amount of time to render the relative vacuum stronger than its weight? Put another way, isn't it the net pressure of all the air molecules flowing under the plane that push up on it to try to fill the vacuum created above the wings? And then isn't that similar to the pressure of the air below a hot-air balloon pushing up on the balloon because more is building up under the balloon? It's as if the plane does with air-motion over time what the balloon does with heat in space/volume. Helicopter rotors might be a better example since their lift literally generates a low-pressure zone above the vehicle while stationary, correct?

"Nature abhors a vacuum," which is why I thought air or water flows downward in the gravity field. I.e. higher pressure moves toward lower pressure to seek equilibrium pressure. I guess this doesn't explain buoyancy, though, since gravity doesn't care about pressure, only density. I guess I got confused because gravity causes the pressure by piling up all the molecules on top of each other in the first place, which causes them to rush into a vacuum with the force of atmospheric pressure. Why is this discussion requiring so many words? It seems like this should be simple to explain.

EDIT: Density = Mass/Unit Volume, Pressure = Force/Unit Area. Two completely different things.[/QUOTE]
 
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  • #12
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Lighter/heavier than the volume of air displaced at a given moment, maybe, but doesn't the plane have to displace a sufficient amount of air in a certain amount of time to render the relative vacuum stronger than its weight? Put another way, isn't it the net pressure of all the air molecules flowing under the plane that push up on it to try to fill the vacuum created above the wings?

Tell that to aircraft flying upside down.
 

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