MadSci Network: Physics

Re: About that 1000000 Lb boat in 100 Lbs of water?

Date: Fri Apr 20 01:41:04 2001
Posted By: Gary Treistman, Undergraduate, Gen. Knowl. Dept., Programming Technologies
Area of science: Physics
ID: 986433308.Ph

The simple answer is yes, 100 lbs of water *could* support a 1000000 Lb
And the reason is counter-intuitive and that is why is hard to comprehend.

Of course the words used to describe the scenario are all loaded with
implicit meaning, and presume you are envisioning the right situation.

So, the usual scenario for this kind of proposition is that you have a
container (capable of supporting 1000000 lbs) whose interior matches the
(bottom and side)contours of the boat.

You then put the boat into this container without any water, with it just
sitting in there.

Finally, you add the 100 lbs of water into the container, between the boat
and the container.
Presuming that the boat's total average density is less than 1gm/cm
density of water), then the boat *will* float on that thin layer of  water.

The statement that the boat must displace an amount of water equal to its
weight is actually incomplete.

What it really means is that the boat would displace an equal weight of
water if that water previously existed in the same volume way in the first
This is an resulting effect, not an a priori cause of the phenomena.

The 100lbs of water is not doing the ultimate supporting of the boat; the
actual support of the boat is being done by the container beneath the 
of water.

The mechanism by which the 100lbs of water acts as an intermediate support
of the boat is:
1. The pressure of the area of the water touching the boat is equal &
opposite at every contact point to the pressure of the bottom/side of the
2. The pressure of the bottom of the boat is equal to the weight of the 
multiplied by its bottom/side surface area.
3. This pressure (which is force/area) is not enough to push a column of
water (of any given cross sectional area) above the sides of the container.
4. Thus the 100lbs of water will rise only to the necessary height to
counter-balance the less-than-water-dense boat.

There are other equivilent explanations for the effect, but this is
basically it

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