Compressing the Uncompressible

While I was reading about the Triton submarine over the last week, it occured to me that it would be possible to test the Triton's pressure hull on dry land, i.e. it was small enough that you could conceivably build a pressure tank big enough to contain it. Then I got to thinking about how much work it would be to pressurize it, and I realized that if you got all the air out it wouldn't take much. You could probably use something like a three inch diameter hydraulic cylinder with a long lever attached. Lean on the end of the lever enough to move the piston an inch or two and you could have all the pressure you needed. Of course the trick is to get all the air out. Any bubbles caught inside the tank could easily absorb all your hand made compression without raising the pressure noticeably.

Then Bobbi mentions traction engines, which is just another name for locomotives. I suspect they are called traction engines to distinguish them from cog engines which use a gear wheel on the engine to push against a rack laid between the rails. If you remember your geometry, a line tangent to a circle intersects the circle at only one point. A steel railroad wheel when it sits on a rail is tangent to the rail, and steel is uncompressible, so the contact is at one point, which we know from geometry has no measurable length. Friction is measured by a coefficient (a number) and the area of contact. If the area of contact is zero, then the friction is zero, no matter what the coefficeint is. If there is no friction, there is no traction, so how can a locomotive pull anything?



Lastly there is Newton's Cradle, the executive toy with five or six steel balls hanging from strings. Look at any explanation and they talk about perfect elastic impacts. Steel is elastic? Well, in a word, yes. From an answer on Yahoo: Actually water is slightly compressible, about the same as steel.