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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I was reading this question and found it interesting. The site I read ventured to say, yes it can. It presented a graphical answer: if you put a thin film around the ship, leaving a thin layer around the ship. Freeze the water around the film and carve out a bathtub. The ship should be floating.
Now, I don't know if I accept this. When the ship is dropped in the tub it displaces its weight of water. This mass of water surrounds the ship and pushes down, and this force works its way underneath the ship and pushes up. If one freezes the water around the ship, and carves out a tub, the "push" from the water around the sides is lost, and the ship might sink.
Imagine two cylinders, with one fitting inside the other. If the layer of water between the two is thin enough, will there be enough "mass" to push down in order to push the bottom of the cylinder up?
For this question I am ignoring electrostatic forces and surface tension. I am just talking about bouyancy.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi,
Yup, you seem to be rediscovering the famous "hydrostatic paradox", which says that a narrow column of fluid indeed exerts the same pressure to it's bottom as a huge one.
It's easiest to consider the forces if you simply think of submerging a brick/cylinder into a tub just slightly larger than it. Now matter how you reduce the area of columns of water around the block by making the tub smaller, you reduce the area they are pushing on as well, and hence the pressure remains constant, and equal to pressure exerted by the block which is always constant (as long as it floats, if you push it in by hand, the side columns will rise to mantain the pressure) - in other words, you can think of the fluid on the floor acting as a sort of a hydraulic press here. In the end it is the floor of the tub that bears the downward force load, not the side columns as one can be led to believe.
Finally, you must never forget that the submerged part of the ship is still exactly the same density as water - thinking that it would sink in a tub reduces to absurd as soon as you realize that water should then sink into water when you pour it in as well...
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Marko wrote ...
.Finally, you must never forget that the submerged part of the ship is still exactly the same density as water - thinking that it would sink in a tub reduces to absurd as soon as you realize that water should then sink into water when you pour it in as well...
Marko
This is incorrect, the submerged part is not the same density as the water.
Think of an iceberg. Is the submerged part of an iceberg the same density as the water displaced? The answer is no, ice is less dense than water.
The total weight (mass) of the ship is the same weight (mass) as the water displaced by the submerged part.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
OK, an old quiz problem:
A boat is floating in a small pond. In the boat are a man and a large rock. The man picks up the rock and drops it overboard. What will happen to the water level in the pond?
I like this one because it's easy to actually demonstrate, in a bucket or a kitchen sink. Been there, done that.
-Rich
[edit] and of course a stationary hovercraft at sea makes the same size depression in the water, whether its lift blowers are on or off.
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Will a battleship float in a bathtub?
