# Thread: mass of an object in water

1. I am coming against some difficulties when trying to determine the mass of a object in the water. I cannot simply weigh the organism because it is too large.

I can determine the exact volume. I originally thought that it was neutrally buoyant and so I planned to determine the volume of the object, and then use the archimedes principle to show that the mass is equal to the mass of the water that the object displaces, which can be worked out through volume.

However, this turns out to be an incorrect assumption. The objects I am studying are actually very slightly negatively buoyant (I am not able to establish how -vely buoyant).

Is there some way of determining density without mass that I haven't managed to come up with yet? or would it be better to just add a coefficient for the negative buoyancy to my equation adapted from the archimedes principle??

I'm really going for accuracy, so not using a 'fudging factor' in my equation would be ideal...

Really hope this isn't a non-starter!!
Also, please be kind with the physics terminology, I am a marine biologist by trade, not a physicist!

Jen

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3. Is this a whale?

I am thinking there must be indexes for various marine animals, where you could work out a close approximation of mass by determining body mass index.

4. If it is neutrally buoyant, then of course, the density of the critter is exactly the same as the density of the water it is floating in, which is a function of the temperature and perhaps salinity if applicable. You could look that up in a table somewhere. But, as you say it is negatively buoyant that won't be exactly right. How accurate of a number do you need?

If you could somehow get an accurate weight of the water in the tank with organism, and the weight sans organism, that would be a way of weighing the organism. That would be hard to do with any kind of accuracy. If you could hoist the whole tank with a crane or hoist with a load cell attached, you might be able to get a pretty good measurement. Would it be better than guesstimating that it has neutral buoyancy? I don't know.

A whale's density would be variable, depending on how much air is in its lungs, and the compression of the air when it dives. Fish can change buoyancy with a swim bladder.

5. That is quite simple, measure its water displacement, and use a spring scale to measure the mass it moves. Then on can know the density and determine the weight
If something has the same exterior general density measuring the mass is best done via axtinction of radiative sources that are shot through it.
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