Modifying the volume modifies the buoyancy of a body?

Submitted by wr0ttit t3_zrgmqk in askscience

I always wanted to ask this, 'cos it puzzles me.

  1. You have a high pressure helium tank, closed, but connected to a large empty balloon, underwater. You open the tank and fill the balloon with a significant volume (should work if enough pressure in the tank). Will the system start floating?
  2. You have a closed tight syringe-like tube with a piston inside, in "compressed" state (occupying low volume), underwater. Inside the tube, there is a small electric engine (powered by a battery) connected to the piston. The weight of the whole system is very low. Starting the engine (i.e. remote controlled) will push the piston towards outside (increasing the volume). Will the tube start floating if enough delta-volume is made by the movement of the piston?
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RobusEtCeleritas t1_j15vg1s wrote

The buoyancy of an object in a fluid depends on the average density of the object and the density of the fluid.

In the situations you're describing, the mass is constant and the volume is changing. So the average density is decreasing as the volume increases, and indeed, if the average density falls below the density of the surrounding fluid, the object will become buoyant.

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drhunny t1_j16jy3m wrote

By the way, this is exactly how submarines used to change depth (Modern subs tend to use a different method). They have sections that can be filled with water or air, and tanks of compressed air. If they need to come up quickly, they let some of the compressed air into those sections, pushing out the water. This is like your syringe example.

A modern sub is more likely to use that process to get almost exactly zero buoyancy, and then change depth by pointing a bit up or down while running the propeller.

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Liveware_Failure t1_j15wowu wrote

Yes to both, but with current materials you'd struggle to get anything more than a tiny amount of bouyancy from the second example.

If you think about it this way, the mass of both objects remains the same, but their volume increases so the average density goes down.

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bdrwr t1_j169kh9 wrote

The buoyant force in a liquid is equal to the weight of the displaced liquid. A bigger volume has the potential to displace more liquid. The answer to both of your questions is yes: in both cases, the volume increases, displacing more water and increasing the buoyant force, while mass remains constant. Eventually the buoyant force surpasses the weight of the object and it'll start floating.

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auraseer t1_j1dmiwt wrote

> You have a high pressure helium tank, closed, but connected to a large empty balloon, underwater. You open the tank and fill the balloon with a significant volume (should work if enough pressure in the tank). Will the system start floating?

If the balloon is big enough, yes.

Exactly the same thing will happen as if you do it in normal air. Imagine hooking up a very high pressure helium tank to a blimp, and inflating the blimp, so that it takes off with the tank attached.

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