Weighing an Object in Water: Does the Bouyant Force Matter?

[phymatter]

if i place a weighing machine at the bottom of the swimming pool and then put a box on it whose density is greater than the iquid , then will it weigh more or less , as compared to on the land ?
i think that it will weigh less , but if its lower surface is in contact with the surface of the swimming pool then how can it experience the bouyant force??
this is troubling me , please help!

 

[HallsofIvy]

Are you suggesting that if you placed a very light, say rubber, cube on the bottom of the swimming pool it would not float? Bouyancy affects the entire object, not just its lower side.

 

[phymatter]

Are you suggesting that if you placed a very light, say rubber, cube on the bottom of the swimming pool it would not float? Bouyancy affects the entire object, not just its lower side.

no , i mean that if we put a very heavy box which rests compleatly on the weighing machine so that there is no exposed area at the bottomof the box where the force due to upward pressure could be exerted , sure the pressure will be exerted from the top , which will cause the weight to increase , the side forces will cancel , but there will be no way for upword bouyant force to act !

 

[russ_watters]

You would need to actually seal the bottom surface to the scale (and the scale would need to be sealed) to eliminate the buoyant force.

 

[sophiecentaur]

If the object had a perfectly flat bottom, with perfect contact with the floor of the tank and vertical sides then there would be no upward vertical forces on it. The weight would measure as the object's weight plus the weight of water on top.

There is a simple experiment to show the same effect, this time with air. Lay a newspaper, spread out, on a table with a ruler half under the paper. Then try to lift the ruler quickly. The excess pressure above will temporarily resist you forcibly - until some air leaks in underneath the paper.

 

[phymatter]

thank you everyone ! this forum rocks! :)

 

[sophiecentaur]

Yes it certainly does and, d'you know another great thing about it? Hardly anybody ever whinges or gets too upset. I've followed some forums which are full of fanciful weeds who don't want to know about real Science at all but they sob sob when that's pointed out.
We're well mature innit.