Why Buoyancy is Imaginary

[DanW9494]

Density of the floating object is irrelevant to a discussion of buoyancy. A cubic foot of lead has exactly the same buoyancy as a cubic foot of foam in water. The buoyancy is determined by the volume.

I'm not strong enough to hold up a cubic foot of lead with it's top at the surface over the side of my boat, it would pull me over. Yet, I would struggle to keep a cubic foot of foam underwater.

If buoyancy is due to the water pressure alone, then why the difference in the work it takes to do that experiment?

 

[DanW9494]

Dan, For your calculations to be completely valid you must select your 2x4 from clear, straight grain wood, with no grain runout, with a uniform growth rate across its width. You did not specify the size of the screw eye and you didn't allow for water absorption in the submerged portion of the stick.

Oh! one other thing, Did you allow for the distribution of mass when you calculated the ballast?

You're right on the first paragraph (the perfect 2x4 that only our grandfathers got to see) and no for the second.

 

[DanW9494]

Roger,
I responded too fast to you without thinking because obviously their weights are different.

But what exactly is going on when we can change the density and buoyancy of a submarine without changing it's displacement, and we can dial in it's depth?

I just keep coming back to that reality.

 

[Todd Smith]

Water in and out

Roger,
I responded too fast to you without thinking because obviously their weights are different.

But what exactly is going on when we can change the density and buoyancy of a submarine without changing it's displacement, and we can dial in it's depth?

I just keep coming back to that reality.

If you think of a submarine as a constant displacement (outer skin of hull), then you are moving water in and out of that hull, making it heavier or lighter.

I think that most people think of a submarine as variable displacement, depending on how much water is in the ballast tanks.

Todd

 

[Bill Roosa]

viva la differance

DanW9494
The force that it takes to push a cubic foot of foam into the water is calculated exactly the same way the force to hold up a cubic foot of lead.

If we constrain ourselves to a cubic foot (1x1x1) with top surface at the water surface in both cases we get the pressure in the water at 1' down =62.4 lb/ft^3

The pressure on the vertical sides does not contribute to forcing the shape upward and all sideways forces cancel out.

For the foam we will assume its' total weight is 1 lb
So weight acts downward with a force of 1 lb and water pressure (acting on the 1x1 lower surface) pushes upward with a force of:
62.4 * 1 * 1 = 62.4 lb

Add the two together (down is negative)
62.4 - 1 = 62.4 lb upward

For lead we get 780ish (I don't recall the density of lead off the top of my head) lb of weight acting downward and the same 62.4 lb of water pressure acting upward. Now when we add the two we get (again with downward neg)
62.4 - 780 = -717.6 downward

Foam weights less, that is why it does not sink. I would have thought you would have figured that out by now.

 

[Bill Roosa]

Subs, varable displacement or variable density?

You can think of them in either way because those two properties are necessary consequences to managing the forces that make a sub work.

To wit:

You take on water (decrease displacement) seal the tank (now density magically went up) and sink.
You open the tank, blow the water out (increase displacement and lower the density) and rise.

With the valves open to the sea the displacement can be changed
With the valves closed to the sea the density has been changed.

But NEITHER of these properties produces a force The are scalar properties not vectors.
What is really going on is you are changing the weight of the sub. If I do that with water (handy, heavy, and easy to move so why not!!) everybody thinks the magical buoyancy is doing it. You could just as easily load the sub with a bucket of buckshot and make it sink. When you get to the bottom you can start letting the buckshot out and the sub would rise. That is how some deep submersibles do it BTW. They don't use ballast tanks they litter the ocean floor with metal weights.

 

[Ross]

Water lines link seems to be broken.

 

[Andy1237]

Simple answer to buoyancy-its MAGIC! and the canvas(dacron) can do miracles

 

[Bill Roosa]

You may be on to something Andy

But I bet it is the buoyancy ferries under the boat holding it up. I mean how could water hold up anything. When I put my hand in it I sink right in.

Perhaps we could catch one and put it on U-tube and disprove the whole "science" myth. What do you think buoyancy ferries like to eat?

 

[Roger Long]

What do you think buoyancy ferries like to eat?

I think it's "fairies" but I know for sure that their favorite food is money.

 

[Bill Roosa]

Well that explains a lot

So there we have it. The root cause of every boating problem.

I suggest we stop calling ourselves boat owners and start calling ourselves buoyancy ferry feeders.

A boat is truly a hole in the water where you throw money.

Since I believe that I cannot overfeed my ferries (and kill them) perhaps there is a "ferry repellent" that could be applied to the hull during hall out. You would no doubt not be able to float the boat with such a coating though so I guess we are stuck with them.

 

[Ross]

Re: Well that explains a lot

Those fairies must be very accomodating. I have seen some boats that were so ugly and neglected that I couldn't understand why they were still on the surface. Sumthin must have been holding them up.

 

[Bill Roosa]

I suspect they are unionized

They certainly are no respecter of boats. I suspect they are operating under union rules as even when I've "paid my dues" they still allow water to enter the boat when I clean the knot-meter sensor.

Perhaps they could be bribed in an emergency to keep the water out of the boat.

 

[therewolf]

So the name describing the force is still Bouyancy or is it Imaginary?

Aviators have gremlins trying to knock us out of the sky. They must be distant relatives of the float fairies, gremlins not the pilots. Similarly, is Lift also being renamed to Imaginary?

 

[jibes138]

Is Lift Imaginary

Yes. An Airplane flies, not because of lift but because the sum of the forces in the vertical direction are not zero. That is until it reaches a cruising altitude, then the sum of the forces in the vertical direction are zero again. The forces in the horizontal direction are usually positive based on thrust of the engines. So lets equate this to a sail. A boat moves forward because the sum of the forces in the direction of desired motion are not zero.

 

[Bill Roosa]

The sum of forces is 0 or she is accelerating

From high school physics class

F=ma
Force equals the mass times the acceleration
It the object is not accelerating (velocity is constant or 0) then "a" is 0 and the result F is also F. This is why things in equilibrium have the sum of all forces equal to 0. They are not accelerating.

For moving boats the forward force from the prop is balanced by the friction force due to water drag on the hull (or the buoyancy ferries working their second job, times are rough all around). So when your bottom is dirty it will produce more drag at speed (drag is proportionals to the velocity squared so it is VERY noticeable) and you will either not go as fast for a given engine power setting or have to increase power to get to the same speed as when she has a clean bottom.
So keep your bottom clean to keep speed up.

When you are stopped in the water and drop the tranny in gear the forces are unbalanced so you get an acceleration till the friction picks up to stop further acceleration. Then velocity is constant.

 

[Bill Roosa]

The girlfriend is bothering me

Sorry folks.
We are having a snow day here and the girlfriend is cooking and other stuff and distracting me.

I should have typed

"Force equals the mass times the acceleration
If the object is not accelerating (velocity is constant or 0) then "a" is 0 and the result F is also 0. This is why things in equilibrium have the sum of all forces equal to 0. They are not accelerating.

 

[jibes138]

Bill I agree and Disagree

Things are not in equilibrium because they are not accelerating. They are not doing many other things also. The state of an object is not defined by the fact that it is not in every other state it could possibly be. "A body at rest stays at rest until acted upon by an outside force." So a body is at equilibrium because that is its' current state. Until it is acted on by a force it will stay that way. If a force is applied to it then it will accelerate or not depending on the direction of the force and opposing forces. For example. A 10 pound block 1 cubic foot sitting on the ground. Put a one pound weight on it and nothing happens other than a bit of compression of the material below the yield point. The same block pushed on the side may move if the friction coefficient is small enough.
Comments?

 

[Will Sail For Food!]

You guys must really be bored, the next thing you know you'll be discussing how there's no such thing as vacuum!

 

[Allan R]

Balderdash, my misses (Mrs.) has a vacuum and uses to clean up my messes.