What liquid should you use to wet a piece of cloth, so that weight of a wet cloth is less than weight of a dry cloth?

The only idea I have is that you should wet the cloth in sunflower oil and then immerse it into water. It won't sink, because the oil won't let the water to soak cloth, so the cloth will float. But it still has its weight (it' putting pressure on the water) and it weighs more because it has oil in it.

So I'm comletely stumped here.

I know, that this question was offered in the university course of molecular physics.


closed as unclear what you're asking by John Rennie, Kyle Kanos, Cosmas Zachos, ZeroTheHero, sammy gerbil May 25 '18 at 16:05

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    $\begingroup$ Seems to me that you're asking if you can add mass to something and make it weigh less than not adding mass, which doesn't make sense to me. $\endgroup$ – Kyle Kanos May 24 '18 at 17:21
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    $\begingroup$ @Kyle Kanos - it makes sense sometimes. If you add some helium into balloon, it will have less weight because of buoyancy force. $\endgroup$ – Anna from Svetlogorsk May 24 '18 at 19:30
  • $\begingroup$ operationally, that might be true but the definition of $W=mg$ shows it isn't . See physics.stackexchange.com/q/130541/25301 $\endgroup$ – Kyle Kanos May 24 '18 at 19:36
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    $\begingroup$ @Kyle Kanos - we are talking about different weight here. I guess, in Englisg the weight I'm talking about is called "apparent weight" and it's defined as a force wich a body exerts on a support or a suspension due to Earth gravitation. $\endgroup$ – Anna from Svetlogorsk May 24 '18 at 19:58
  • $\begingroup$ When you have a strong enough magnetic field underneath it. Water is paramagnetic. $\endgroup$ – Arvin Singh May 26 '18 at 14:53

Supposing that the question is about a cloth on a table (surrounded by atmosphere). There exist no liquid less dense then air. So if you wet the cloth with any liquid it will be heavier. But, if you were to "wet" the cloat in Helium gas (in the process changing all the air in the cloth to Helium) it would then become lighter than it was before.

  • $\begingroup$ Thanks. I've had this idea, but then I've thought that liquid helium would be still more dense than air at normal conditions. Or am I wrong? $\endgroup$ – Anna from Svetlogorsk May 24 '18 at 17:13
  • $\begingroup$ You are right that liquid helium is denser than air. But I wrote Helium gas, which is lighter than air. The concept of "wetting" a cloth might include wetting it with a gas. $\endgroup$ – Maxter May 24 '18 at 17:17
  • $\begingroup$ @Maxter - You will not be able to have this gaseous helium stick to the cloth unless you make a ballon out of an impermeable cloth. $\endgroup$ – freecharly May 24 '18 at 17:52
  • $\begingroup$ @AnnafromSvetlogorsk - Anna, this must be kind of a trick question. If you add any mass to the cloth, the cloth can only become heavier not lighter. $\endgroup$ – freecharly May 24 '18 at 17:54
  • $\begingroup$ @freecharly - Helium was just an exemple. If the cloth can trap air molecule, it can most probably trap some lighter gas as well. Maybe not as light as helium, but certainly lighter than air. $\endgroup$ – Maxter May 25 '18 at 17:42
  • If you're talking about apparent weight then a solution like yours (or Maxter's answer, if it could be done) would work.
  • If you mean simply weight then, as many pointed out, there's no way a "wet cloth" can weigh less than a dry one, because mass has been added to the cloth, thus the gravitational force acting on it won't decrease.

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