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Suppose I had two large containers with 1 cubic meter size. The containers are identical on the outside. The containers are floating in microgravity very close to my space ship.

One was empty but had thick reinforced steel walls so that its mass was 600 kg, while the other had thinner aluminum walls but was about half full, say 500 kg of water and 100 kg dry mass.

I have limited oxygen and need to quickly ascertain which container has water for my ship's solar-powered electrolysis system.

What kinds of tests could be done to cubes, to choose the one with the water? The containers are bulky but meant to be moved around by astronauts in microgravity, they have several hand-holds, in corners, sides, and faces, so I have flexibility in where I grab it.

The ship also has plenty of hand-holds on the outside, and cubes are floating roughly a meter away.

I first think that I can "slosh" it but then realized that I don't really have a good understanding what sloshing means in microgravity. It's not the same as on Earth where you know the liquid/air line is horizontal and the water is the "bottom". So, can I do sloshing test in microgravity for identification of containers?

Are there better ways to manipulate the two containers by using less force that would give me either visual or tactile clues as forces through the handles and my gloves to notice which one has the water?

Remember: I'm running out of oxygen, I want to be clever and not run out of oxygen before I can choose the right container to make more oxygen!

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    $\begingroup$ What are you talking about? What tanks? $\endgroup$ Nov 18 '18 at 21:25
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    $\begingroup$ What kind of fuel tank in space is small enough to be shaken by a human, even under ideal circumstances? $\endgroup$ Nov 18 '18 at 21:44
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    $\begingroup$ He potentially could shake two containers in order to find out which one is full and which one is empty, simply due to noticing the difference in momentum. But its pretty much impractical for anything big enough to resemble a tank. You can surely do it with a 1L water bottle and maybe even significantly bigger tanks, but to what end? Its not like you could EVA and shake the fuel tank to find out if there is something left in it. $\endgroup$
    – Polygnome
    Nov 18 '18 at 21:54
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    $\begingroup$ Welcome to Space! There is a device called an inertial balance which can measure mass without any gravity. The object in question is attached to a platform on the end of spring arms, is perturbed, and the period of oscillation measured: arborsci.com/pub/media/catalog/product/i/n/… However, it's not a good choice for a tank, as all of the plumbing would have to be disconnected. $\endgroup$
    – DrSheldon
    Nov 18 '18 at 23:36
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    $\begingroup$ Voting to re-open now that I've added some more details. With the OP's consent hopefully we can now think about this much more detailed and defined microgravity sloshing question. $\endgroup$
    – uhoh
    Nov 19 '18 at 10:01
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Yes. First pull the tank gently in some direction until it has moved at least 1m. Now pull it in the opposite direction until it comes to rest. The tank with the liquid will require much less force initially to deccelerate it, but then will need more force (or may even start moving again after stopping) later. The solid tank will need a steadier force.

Explanation: the first pull ensures that the liquid is on the "back" side of the tank. When you then pull in the opposite direction, the liquid will initially keep moving, while the tank decelerates, so you will only be working against 100kg of inertia instead of 600kg. When the liquid hits the opposite side of the tank it will change the dynamics. The solid tank will have none of this, and is just a 600kg mass firmly attached to the handles.

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