Physics Question (1 Viewer)

[Bill Catherall]

Of course, IF there is an upward buoyant force and if it were less than the weight of the pound of feathers than, naturally they would not float, and yes a pound of feathers in air would have more mass than a pound of feathers in a vacuum. But again, the teacher would be assuming that the feathers are more buoyant than the nails.

I could just as easily, and just as (in)correctly state that the nails are more buoyant and therefore a pound of nails in air has more mass than a pound of nails in a vacuum. So the nails are "heavier."

Or if they have equal buoyancy (another assumption that isn't more invalid than the first) then the pound of nails would have the same amount of mass as the pound of feathers regardless of the presence of air. So they "weigh" the same.

Where's the proof that feathers are more buoyant than nails?

 

[RobertR]

Where's the proof that feathers are more buoyant than nails?

If you look at a free body diagram of the feathers vs. the nails, the bottom of the feathers would have a greater surface area exposed to upward-acting air pressure than the nails. Therefore, the net force due to air pressure (and therefore the buoyancy force) would be different.

 

[Bill Catherall]

If you look at a free body diagram of the feathers vs. the nails, the bottom of the feathers would have a greater surface area exposed to upward-acting air pressure than the nails. Therefore, the net force due to air pressure (and therefore the buoyancy force) would be different.

You're saying the feather has a greater surface area on the bottom than the top? I don't think so. The air pressure on the bottom would be equalized by the air pressure on the top.

Next.

 

[RobertR]

You're saying the feather has a greater surface area on the bottom than the top?

No, that is not what I said. The bottom of the feathers has a greater surface area exposed to upward air pressure than the nails. Hence the different buoyancy factor. If you doubt this, ask why feathers fall more slowly in air than nails do.

 

[BrianW]

You're saying the feather has a greater surface area on the bottom than the top?

I've been trying to invent just such a device. When I do, it'll make me rich!

 

[Bill Catherall]

No, that is not what I said. The bottom of the feathers has a greater surface area exposed to upward air pressure than the nails. Hence the different buoyancy factor. If you doubt this, ask why feathers fall more slowly in air than nails do.

Like I said, the top surface area of the feather cancels out the bottom surface area. But that doesn't matter anyway.

The surface area has nothing to do with buoyancy. Buoyancy is dependant on volume and density. What you're talking about is air resistance. Feathers fall slower because they have greater drag, not buoyancy. I'm looking for proof that feathers are more buoyant than nails in air. It has nothing to do with their rate of decent.

Now, I just know someone is going to say, "Oh, but in order to have an equal amount of mass as the nails you have to have a much larger volume. Therefore, feathers are less dense and are more buoyant." Unfortunately, that would be a wrong assumption to make as well, because most of the volume of a feather is taken up by air. So removing air from the volume, what's the density and how does it compare to a nail?

 

[RobertR]

Like I said, the top surface area of the feather cancels out the bottom surface area. But that doesn't matter anyway.

But the idea is that there's less cancellation with the pile of nails, therefore the net downward force of air on the nails is slightly greater.

 

[DaveGTP]

If the air isn't moving, there wouldn't be any difference in the forces. It seems like technically since air pressure in the atmosphere is the weight of the air pressing downward towards the center of gravity in the Earth, by THAT logic, the feathers would be heavier, not lighter, because of the air pressure! (sorry for the run on sentence).

Anyway, if you are talking scales, scales are designed already IN air pressure. Therefore already adjusted for air pressure, otherwise they would show a weight when empty.

 

[RobertR]

It seems that the way to test it would be to weigh the feathers in atmosphere vs. in a vacuum. If the weight reading increases slightly, then the explnation is reasonably clear.

 

[Bill Catherall]

It seems like technically since air pressure in the atmosphere is the weight of the air pressing downward towards the center of gravity in the Earth, by THAT logic, the feathers would be heavier, not lighter, because of the air pressure!

No. Air pressure presses on all sides of an object. If not you would collapse from the weight of the air on you. There is equal amount of pressure pushing up on all objects as there is pushing down. This is the case for all fluids. Pressure exerts force in all directions. It has nothing to do with gravity.

Atmospheric pressures have nothing to do with buoyancy, other than changing the density of the fluid, since density changes with pressure. It exerts only inward forces (or outward if it's a negative pressure).

 

[BrianW]

It seems like technically since air pressure in the atmosphere is the weight of the air pressing downward towards the center of gravity in the Earth, by THAT logic, the feathers would be heavier, not lighter, because of the air pressure!

That's a bad assumption to make. Although the pressure of a fluid is certainly affected by depth, in the absense of non-Brownian currents, the pressure of a fluid at any given point is the same in all directions. After all, the force of the air being pulled down is countered by the force of the air below pushing back up.


Edit: I don't know what just happened here, but I swear Bill's post wasn't here when I posted, even though my post indicates that it arrived more than a half hour after his. Maybe I should go back to the posting-while-drunk thread.

 

[Holadem]

I agree that air pressure and drag for that matter have squat to do with this.

So removing air from the volume, what's the density and how does it compare to a nail?

I haven't a clue what feathers are made of, but I think it's safe to assume that feather - air = some substance (let's call it pure feather ) that is still less dense than the metal nails are made of.

[EDIT]It just occured to me that the convoluted explanation I had posted was not needed. I believe anyone who has participated in this thread will reach the same conclusion if we agree with the premise that pure feather is less dense than the nails.

--
Holadem

 

[Bill Catherall]

I believe anyone who has participated in this thread will reach the same conclusion if we agree with the premise that pure feather is less dense than the nails.

True, we'd have to agree on that assumption. But since it's only an assumption that can't be proven without data, it's still inconclusive. I could also just as easily assume it's more dense or has an equal density.

 

[DaveGTP]

No. Air pressure presses on all sides of an object. If not you would collapse from the weight of the air on you. There is equal amount of pressure pushing up on all objects as there is pushing down. This is the case for all fluids. Pressure exerts force in all directions. It has nothing to do with gravity.

Sorry guys, I had a too-long post worked out and hacked it down to the basics without being careful. Air pressure does indeed come in in all directions. What I meant to say was IF you were to (Erroneously!) assign a direction to the force, it would be down, if anything (definitely not up). Just trying to point out fallacies in the air pressure-upwards-force theory.

 

[Cees Alons]

Aren't you folks complicating the discussion too much? The whole point of air pressure at the bottom versus air pressure at the top equals the "a body immersed in a fluid..." problem as solved aready by, uhmmm..., Eureka!

Yes, a less denser body when weighed in a gas or fluid environment (like air, or water or oil) weighs less than a more denser body (all weigh less than they do in vacuum) - if measured in a forcefield at exactly the same other circumstances. Explaining why Archimedes was right isn't necessary here anymore. The upward lift is the weight of the displaced gas (or fluid).

But whatever:

My physics teacher claims that a pound of feathers weighs more than a pound of nails.

(emphasis by me) still isn't true. They would weigh less if you want to take those sort of things into account.

Personally, I think that the teacher's error was to intuitively think that "weighs more" meant "is absolutely more". Which under the circumstances is absolutely wrong.


Cees

 

[Rob Gardiner]

Personally, I think that the teacher's error was to intuitively think that "weighs more" meant "is absolutely more". Which under the circumstances is absolutely wrong.

Exactly. The teacher is confusing weight with mass.