[ODE] objects in water

[Bob Dowland]

> In fact, once you go further down, the object may start to compress 
> because of the high pressure, and thus its density goes up, 
> and it gets 
> less buoyancy further down. Divers can feel this at 10 meters!

Second that - in fact a diver begins to feel hydrostatic pressure on the eardrums at 2-3 meters. By 10 the pressure is double wrt surface (10m == 1 atmosphere (water) + 1 (air)). In fact even on a 40 m dive it is the ten closest to the surface which are the most dangerous due to pressure gradient. 

But this (hydrostatic - ie due to depth) pressure acts normal to the surface (all over - isotropic) so the sum is zero and there is no direct influence on the motion of the centre of mass. Buoyancy, on the other hand, is just about relative density of the object and the fluid it's displacing, this can only change for a given object if the pressure can cause a change in density - as it does for eg. on an air bag.

Interestingly enough I believe fish have an (air?) bladder which works by similar principles - muscular contractions expand and contract it so that the fish either climbs or descends due to the water it's displacing.

:)







> -----Original Message-----
> From: Jon Watte [mailto:hplus-ode at mindcontrol.org]
> Sent: 17 January 2005 16:39
> To: Tyler Streeter; ode at q12.org
> Subject: RE: [ODE] objects in water
> 
> 
> 
> > Here's a question: does penetration depth matter for buoyancy
> > calculations?  I'm guessing that buoyant objects deep 
> underwater would
> > have more buoyant forces than objects close to the water's surface. 
> > Even so, it's probably not worth the effort to simulate this in most
> > real-time applications.
> 
> That's not true, except when you're partially intersecting 
> the surface.
> 
> When something is partially in water, and partially above it, it only 
> has as much floatation force as the part that's within the 
> water. Once 
> the thing is fully submerged, it exerts the maximum 
> floatation it can, 
> and pushing it further down won't exert more force (it's not 
> a spring!)
> 
> In fact, once you go further down, the object may start to compress 
> because of the high pressure, and thus its density goes up, 
> and it gets 
> less buoyancy further down. Divers can feel this at 10 meters!
> 
> Cheers,
> 
> 			/ h+
> 
> 
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