The physics of underwater "weight"

Rathark · Post by **Rathark** » 2002-08-18 07:11am

I came up with this question months ago while working on one of my amateurish CGI cartoons. If a right whale "gently" sat on a dolphin underwater - for less than a minute - would the dolphin be crushed to death, or merely trapped? Assume that the dolphin will not have time to drown.

Obviously, the laws of momentum and kinetic energy will still apply. 80 tons moving in any direction is going to cause some damage unless it is moving reeeeaaalllyyyy ssssllllllooooowwwllllyyyyy. I was wondering how slowly, though - it seems that in real life, the dolphin would either be crushed or have plenty time to escape.

Not that I'm going to restrict my cartooniverse to "real" physics. Heaven forbid.

MKSheppard · Post by **MKSheppard** » 2002-08-18 08:46am

Rathark wrote:I came up with this question months ago while working on one of my amateurish CGI cartoons. If a right whale "gently" sat on a dolphin underwater - for less than a minute - would the dolphin be crushed to death, or merely trapped? Assume that the dolphin will not have time to drown.

http://www.dolphinsex.org

Rathark · Post by **Rathark** » 2002-08-18 09:25am

AAAAAAAAAAARRRRRRRRRGGGGGGHHHHHHHH!!!!!!!!!

No, it has nothing do do with that. Think Tex Avery.

The first thing to come into your mind. Hmmmmm .....

MKSheppard · Post by **MKSheppard** » 2002-08-18 09:26am

Rathark wrote:AAAAAAAAAAARRRRRRRRRGGGGGGHHHHHHHH!!!!!!!!!

HHEHEHEHEHEHAHAHHAHAHAHBWHAHHHHAHAHAHA!

MKSheppard · Post by **MKSheppard** » 2002-08-18 09:27am

Rathark wrote:AAAAAAAAAAARRRRRRRRRGGGGGGHHHHHHHH!!!!!!!!!

HHEHEHEHEHEHAHAHHAHAHAHBWHAHHHHAHAHAHA!

XaLEv · Post by **XaLEv** » 2002-08-18 10:14am

ROFL @ MKSheppard

aerius · Post by **aerius** » 2002-08-18 08:48pm

MKSheppard wrote:
www.dolphinsex.org

Thank you for sharing, that was most informative and entertaining. ROFL

Rathark · Post by **Rathark** » 2002-08-18 08:57pm

Actually, I was thinking of adding a similar scene involving a large, heavy submarine. The physical results of that are going to be somewhat different, I suspect.

I wonder if Sheppard can find an appropriate link for this one ...

Wicked Pilot · Post by **Wicked Pilot** » 2002-08-18 09:06pm

No the dolphin would probably not be crushed. I don't know this for certain, but a whale surely has positive boyancy. Assuming this is correct, the whale's weight is being supported by the water that it swims in. Therefore, the net force of the whale is pointed up toward the surface. I hope this answers your question.

Rathark · Post by **Rathark** » 2002-08-19 02:18am

I'd presume that a 200 ton steel ton submarine moving in any direction is going to crush you against anything (unless you borrow yourself into the sand). An 80 ton whale has a yielding rubbery surface, so you'd stand a better chance, whatever that means. The factors here seem to be the hardness of the surface, the bouyancy and the speed that it is moving through water. A submarine has such a solid, unyielding surface that I would presume that bouyancy and slowness would be cancelled out. I can't see how a submarine could press you against the seabed or a rock wall and not crush you outright, regardless of how slow it is moving. A whale seems to be on the borderline here. A right whale would certainly have positive bouyancy, more so than almost any other species. If she is moving at only 0.316 metres per second, at 80 tons, then that's 4000 joules of kinetic energy - the equivalent of being hit by a car moving at 10 km/h. The momentum, however, would be equivalent to being hit by a car travelling at 90 km/h - not a pretty sight. What would be the result here? Would the momentum / KE be spread out over the whale's body? Would the momentum / KE be more focused on the point of impact if it's a submarine, which would obviously cause more damage?

And if a whale "bumped" you sideways at that slow speed, what does this do to your momentum if you're only 1000th of her mass? I know water resistence comes into play here, but still ...

(PS: I'M FINALLY A YOUNGLING!!!! YAYYYY!!!!!)

Rathark · Post by **Rathark** » 2002-08-21 02:10am

Sorry, I forgot to consider what was most obvious.

If the whale doesn't slow down until she can move no further against the seabed, then she will DEFINITELY crush you, regardless of how slow she's moving.

If, however, she deliberately slows to a halt once you're trapped, then I guess USAF Ace's positive bouyancy theory may (temporarily) save your life, until you begin to drown or run out of oxygen.

On a mildly related subject, does anyone know exactly how the laws of momentum operate in a fluid environment? Surely this would be an essential topic for submariners and naval engineers.

Graeme Dice · Post by **Graeme Dice** » 2002-08-21 02:40am

[quote="USAF Ace"]No the dolphin would probably not be crushed. I don't know this for certain, but a whale surely has positive boyancy.[/qoute]
I would certainly hope that no marine mammal had positive buoyancy, because that would make it nearly impossible for them to swim at a single depth for any length of time.

Post by **Darth Wong** » 2002-08-21 03:06am

Rathark wrote:If, however, she deliberately slows to a halt once you're trapped, then I guess USAF Ace's positive bouyancy theory may (temporarily) save your life, until you begin to drown or run out of oxygen.

A whale must have fairly neutral buoyancy, otherwise it would be difficult to dive or return to the surface. This means that it does not exert any "weight" on an object beneath it.

On a mildly related subject, does anyone know exactly how the laws of momentum operate in a fluid environment? Surely this would be an essential topic for submariners and naval engineers.

The same way they operate in any other environment. If a whale slams into you, some of its momentum will transfer to you. However, most of its momentum would remain with the whale, for the obvious reason that the whale's not going to slow down too much from hitting little ol' you.

Post by **Sea Skimmer** » 2002-08-21 03:36am

Darth Wong wrote:
Rathark wrote:If, however, she deliberately slows to a halt once you're trapped, then I guess USAF Ace's positive bouyancy theory may (temporarily) save your life, until you begin to drown or run out of oxygen.
A whale must have fairly neutral buoyancy, otherwise it would be difficult to dive or return to the surface. This means that it does not exert any "weight" on an object beneath it.

IIRC whales have the ability to alter their buoyancy by heating or cooling parts of there bodies

Rathark · Post by **Rathark** » 2002-08-21 05:22am

Here's a relevent quote I found using Google.

Source:

Daniel K. Odell (odell@pegasus.cc.ucf.edu)
Thu, 30 Jan 1997 21:00:34 -0500 (EST)

Whales are nearly neutrally bouyant (they float low in
the water)but they still have a slight positive bouyancy (blubber floats).
So, they have to exert some effort to dive. Exactly how much effort is
not known. As they descend the air in the lungs compresses and they
become less positively bouyant. The closer they are to neutral bouyancy,
the less energy they need to expend to dive and surface. Remember - this
is somewhat speculative since it hasn't been measured on free-ranging
whales.

What's really shocking about all this is:

1. My cartoon is more physically possible than I first thought. That's probably breaking the rules of the Cartooniverse or something (ie. "If a character is suspended in midair, s/he will not fall until s/he notices that s/he is suspended in midair").

2. Being sat on by a whale is actually less harmful than being sat on by an anorexic supermodel.

StarDestroyer.Net BBS

The physics of underwater "weight"

The physics of underwater "weight"

Re: The physics of underwater "weight"

Re: The physics of underwater "weight"