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I have difficulties explaining this experience: Principe d'Archimède dans le vide.

An object in water does not float. We remove the air on top of the water with a pump, and when the air reached a certain (which one?) low pressure, the object move up?

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The simplest answer is probably that there is a cavity in the object that is filled with air. As the pressure is reduced, the air can expand. This changes the volume of the object slightly, until it's sufficiently large that the weight of the displaced liquid is greater than the weight of the object.

If the object does not change size, this effect could not occur. Water is almost incompressible: and if it was compressible, it would get less dense as the pressure above it decreases. This would mean that the weight of the displaced liquid would decrease.

There is a similar well-known experiment I did as a child with a piece of orange peel: if you add just the right amount of weight to it so it barely floats, and you put it in a closed plastic bottle, then you can squeeze the bottle and cause your "submarine" to sink (because air pockets in the peel are compressed).

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  • $\begingroup$ Not a fluids guy, but wouldn't removal of the air reduce the downward force on the submerged object, which would normally be in addition to the weight of the object and the weight of the water above it? $\endgroup$
    – Bob D
    Commented Dec 21, 2019 at 18:48
  • $\begingroup$ @BobD no - it’s only the difference in pressure between top and bottom of the object that matters. And only the weight of the liquid and volume of the object come into that equation. $\endgroup$
    – Floris
    Commented Dec 21, 2019 at 18:49
  • $\begingroup$ So removing the air above the surface reduces the pressure above and below the object by the same amount, so the difference is unchanged. Correct? $\endgroup$
    – Bob D
    Commented Dec 21, 2019 at 19:18
  • $\begingroup$ @BobD yes that’s right $\endgroup$
    – Floris
    Commented Dec 22, 2019 at 0:12
  • $\begingroup$ Thank you very much. $\endgroup$
    – Bob D
    Commented Dec 22, 2019 at 0:13

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