Floating Battleship poll

[FireGarden]

The floating a battleship in a gallon of water idea seemed very wrong to me to begin with. I had to change my mind, I'm interested in what convinces other people. Hence the poll


I had to conduct an experiment. None of the explanations in the thread actually convinced me, and I preferred to experiment ahead of reading the physics links that were provided.

If anyone's interested
The experiment involved a glass and a flask that was only a little bigger, but big enough to fit the whole glass. The glass weighed about 300g. I filled the flask with water and put the empty glass in until it floated. I weighed the water left in the flask, it was 210g. Less than the weight of the glass by enough to eliminate measuring errors. However, I do feel the need to try it again. I am that turned about by the result!


Oh, the other thread was "Is Marilyn wrong again?"
http://www.randi.org/vbulletin/showthread.php?s=&threadid=41725

 

[Benguin]

I was happy with the physics, but I'm not satisified it is possible to reproduce.

I'm an engineer, not a scientist and that puts me in the 'it's a nice idea, but not on your nelly' camp.

 

[FireGarden]

I meant the basic principle of floating something in a volume of water that weighs less than the something, not literally a battleship in a gallon of water. That may well be impracticle, but in principle it could be done.

Just the 300g glass floating in 210g of water is surprising to me, though the battleship scenario is certainly more exciting!

 

[SGT]

The floating a battleship in a gallon of water idea seemed very wrong to me to begin with. I had to change my mind, I'm interested in what convinces other people. Hence the poll


I had to conduct an experiment. None of the explanations in the thread actually convinced me, and I preferred to experiment ahead of reading the physics links that were provided.

If anyone's interested
The experiment involved a glass and a flask that was only a little bigger, but big enough to fit the whole glass. The glass weighed about 300g. I filled the flask with water and put the empty glass in until it floated. I weighed the water left in the flask, it was 210g. Less than the weight of the glass by enough to eliminate measuring errors. However, I do feel the need to try it again. I am that turned about by the result!


Oh, the other thread was "Is Marilyn wrong again?"
http://www.randi.org/vbulletin/showthread.php?s=&threadid=41725

You don't really have to spill water to test the claim. You can start with the 210 g. I don't know haw much water there is below the glass when it floats, but if it is more than 1 or 2 mm, you can start with less than 210 g (perhaps 100 g) of water and achieve flotation. Would you do the experiment and report it to us?

 

[Benguin]

I meant the basic principle of floating something in a volume of water that weighs less than the something, not literally a battleship in a gallon of water. That may well be impracticle, but in principle it could be done.

Just the 300g glass floating in 210g of water is surprising to me, though the battleship scenario is certainly more exciting!

Well yes, but I don't think we are capable at this time of constructing something within the necessary tolerances or controlling the list and trim of the ship adequately to prevent grounding.

The theoretical physics is fine though.

Just for the sake of being ridiculously pedantic!

 

[Sindai]

I actually swung back and forth while reading the first few posts in the thread, then I finally untangled everything.

 

[Yahweh]

I meant the basic principle of floating something in a volume of water that weighs less than the something, not literally a battleship in a gallon of water. That may well be impracticle, but in principle it could be done.

Just the 300g glass floating in 210g of water is surprising to me, though the battleship scenario is certainly more exciting!

So how much mass until the object cease to float in the 210g of water?

 

[FireGarden]

SGT

you can start with less than 210 g (perhaps 100 g) of water and achieve flotation. Would you do the experiment and report it to us?

A nice excuse to try the experiment again!

This time I began with the glass in an empty flask and poured water between flask and glass until the glass began to float.

80g of water managed to float the 300g glass.



I just completed the above, logged on to post the results, and now find Yahweh has another experiment!

I'm off to get some sleep. Maybe I'll play a bit more tomorrow!
But I'm sure the extra weight would displace more water out of the flask since the glass would have to sink deeper. (remember 210g is how much water was left when the glass stopped sinking) And 80g of water doesn't leave much of a gap between the glass and the bottom.

I'm sure there are many possible combinations of how much will float how much, the question is how to get the biggest ratio! I need to find a practicle way of making the glass a little bigger or the inside of the flask a little smaller.

 

[SGT]

So how much mass until the object cease to float in the 210g of water?

It depends of the geometry of the object and the container. Theoretically you could float an object with several tons in 210 g of water.

 

[LucyR]

The experiment involved a glass and a flask that was only a little bigger, but big enough to fit the whole glass. The glass weighed about 300g. I filled the flask with water and put the empty glass in until it floated. I weighed the water left in the flask, it was 210g. Less than the weight of the glass by enough to eliminate measuring errors. However, I do feel the need to try it again. I am that turned about by the result!

This is not so surprising. Remember that the buoyant force on an object is equal to the weight of the fluid its volume displaces. That does not say that you need that much fluid to make the object float.

Imagine a big body of still water. Further imagine a cube of water somewhere below the surface. What does this cube do? Well nothing of course, it just sits there suspended. This means that the net force on it is zero. This in turn means that the difference in the forces associated with water pressure on the top and bottom surfaces must exactly equal the weight of the cube. Now imagine the same cube in a body of water that is only slightly larger than the cube itself. What happens? Again it is suspended. This proves you can float something in an arbitrary amount of water (obviously ignoring practical difficulties such as you’d encounter in the ridiculous battleship example).

I think part of the problem people have is a misconception of pressure. Pressure is defined as force per unit area. Using the thought experiment above it is trivial to show that the only spatial dimension on which pressure in a fluid depends is depth. You can generate an enormous pressure at the base of tall column of water, regardless of how wide it is.

Another useful illustration is to consider a number of vessels of many different sizes and shapes between which water can freely move (see figure). Do you expect the water level to be different in each vessel? No of course not. It will be the same in each. This means the pressure is uniform regardless of the size or shape of the vessel.

 

[SGT]

SGT

A nice excuse to try the experiment again!

This time I began with the glass in an empty flask and poured water between flask and glass until the glass began to float.

80g of water managed to float the 300g glass.



I just completed the above, logged on to post the results, and now find Yahweh has another experiment!

I'm off to get some sleep. Maybe I'll play a bit more tomorrow!
But I'm sure the extra weight would displace more water out of the flask since the glass would have to sink deeper. (remember 210g is how much water was left when the glass stopped sinking) And 80g of water doesn't leave much of a gap between the glass and the bottom.

I'm sure there are many possible combinations of how much will float how much, the question is how to get the biggest ratio! I need to find a practicle way of making the glass a little bigger or the inside of the flask a little smaller.

If you don't mind ruining your glass, you can paint it with enamel.

 

[Benguin]

You could chocolate coat it, then just melt it off after the experiment.

Or lick it clean.

 

[FireGarden]

LucyR,
Well, I found this surprising. And I'm not the only one.

I'm familiar, of course, with the principle of the lever and pullies. So I know that it's possible to lift a weight with a fraction of that weight as a "counterbalance". It's just that I couldn't see how it applied here.

The way pressure works is not so intuitive to me. I found that this was a useful explanation, from the link Zakur provided in the other thread.

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&th=44d27f6b91e1607&seekm=1308@hao.UUCP&frame=off

I think part of the trouble is a instinctual belief in 'conservation of force'. We feel since there is be 100,000 tons of force holding up the ship, there must me 100,000 tons of something around applying the force. This just ain't so. As another author so nicely pointed out, the important concept is leverage; which allows a small force (the weight of the remaining water) to be translated into a large force (the 100,000 tons of the battleship).

As to the Archimedes' principle, you place to much weight (excuse me) on the word displaced. Imagine our battleship floating in the ocean once again, and suppose we wish to figure out the forces applied to it by the surrounding water. One could do the proper vector intergals over the surface of the ship, but there is a much neater method. Imagine replacing the ship with a volume of water shaped like the portion of the ship below the water-line. Since this would result in just a flat expanse of water, this replacement shouldn't cause any re-arraignment of the water. Thus the net force on the replacement volume of water (and thus on the battleship it replaced) should be exactly equal to the volume's weight. Notice this argument does not require the replacement volume of water to actually exist, it only uses it as an imaginary artifice.

bjornart quoted part of the above, saying that it set him straight. In fact I saw a few people posting that they'd changed their minds because of the arguments provided, but no-one who said that they did an experiment. So I made this poll. I now know that there's at least one other person out there who experimented. So it's been worth it!



SGT
Nope, no enamel!
In fact no more experiments. I'm done for now.

Benguin
Mmmm Chocolate.... Well maybe!