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Posted by Mike Bilow on September 13, 2000 at
04:59:49:
In Reply to: You did not read B. Hannigan's explanations (more)
posted by Bruno on September 13, 2000 at 03:02:27:
Posted from Host: prov05-pm3.intap.net (208.131.219.197)
Partly right, but partly wrong. Deceleration occurs because of the application of force (this is Newton's Second Law of Motion), and pressure is by definition force per unit area. So the only way to decelerate something is to apply pressure to it.
A watch rated to 10 ATM is rated for static pressure, meaning that it is slowly pressurized up to the rated pressure and just made to sit there for a while. Simple variations on this idea, such as pulsing the pressure up and down, can cause failures of seals. Indeed, this slight pressurizing and depressurizing cyclical pattern is what causes metal fatigue on aircraft. This is, interestingly, the same thing a watch experiences when worn by a swimmer.
If a watch is suddenly subjected to high pressure nearly instantaneously, without being brought up to that pressure gradually as it would be in a static test, then this has as much a different effect as throwing the watch against the floor as distinct from gently lowering it to the floor. If a watch is going 190 MPH when it hits the water, I seriously doubt that the seals are going to hold out. Exactly what part of the watch ruptures first may be hard to predict, but water resistance is very likely going to suffer.
-- Mike
: First, what kills people when jumping from a bridge is decelaration (negative
acceleration), not pressure.
: A body (or something with the same Cx - aerodinamic coefficient) would hit water at a speed inferior to 300 km/h (190 mph) whitch is the speed of a freejump. At that speed, dynamic pressure would be equivalent to 83.3 meters of water = 8.33 atm (300,000/3600 = 83.3 m/s).
: Your 10ATM rated watch would not leak but you wouldn't survive. Of course, the decelaration would maybe kill the movement but water wouldn't enter.
: Regs.
: Bruno
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