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Rookie question

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Rookie question

Postby buccaneer » Wed Feb 08, 2017 11:03 am

After 12 years of panning, I have a rookie question. This is something that I thought about when I first started prospecting and was too shy to ask a dumb question. Now after so many trips to the creek I still ponder the answer. We all know things weigh less in water then in the air, so, while panning and working the gold to the bottom of the pan, will the gold in the pan be pulled down by gravity faster with the pan up out of the creek then with the pan under water. :shock: :?:
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Re: Rookie question

Postby sdbridges3 » Wed Feb 08, 2017 12:12 pm

Don't know the specific answer to your question about weight in air versus water but good question because it relates directly to technique. I learned my technique from a professional panner in Georgia. I say "professional" because he is a world record holder in speed panning but teaches the same basic technique for regular panning. His method requires keeping the pan under water all the time you are processing in order to not let any gold escape the pan. Someone else may have some different thoughts on this...

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Re: Rookie question

Postby sanman » Wed Feb 08, 2017 2:49 pm

Good science question! It’s because of upthrust (also called buoyancy). You may have heard of a guy called Archimedes (he yelled “Eureka” when he got in the bath once), he discovered a law of physics that we now call Archimedes principle, this law is:

Any object, completely or partly put in fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

What this means is when you put the brick in the water it takes up space that previously had water there (if you put a brick in a bucket of water the water level will rise slightly because the water has to go somewhere), this water has been ‘displaced’ by the brick. The volume of the water that has been displaced is the same as the volume of the brick. We know how much that bit of water weighs because we know how dense water is (density is a measure of how close together the molecules in something are).

What Archimedes found out was that the weight of this bit of water was the same as the force that the water makes on the brick to push it upwards. Above water, or in the air the same thing is happening, but this time the brick is displacing air, not water. The bit of air it displaces has the same volume as the bit of water from before, but it is much less dense (molecules in air are much further apart than molecules in water) so the bit of air weighs much less than the bit of water from before.

Like Archimedes said the upward force is equal to the weight, so because the water displaced weighs more than the air displaced above water the upwards force from water is bigger than the upwards force from air. So if you hold the brick in water there is a bigger upwards force from the water helping you hold it up so it feels lighter. When you hold it above water the upwards force from the air is smaller so it’s not helping you as much so the brick feels heavier.
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Re: Rookie question

Postby Darth Placer » Wed Feb 08, 2017 5:39 pm

I think if there is water in the pan... whether the pan is in or out of the water it won't make a difference.

It may make the pan feel lighter in the hand if under the water though.
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Re: Rookie question

Postby Gold Seeker » Wed Feb 08, 2017 6:50 pm

Darth Placer wrote:I think if there is water in the pan... whether the pan is in or out of the water it won't make a difference.

It may make the pan feel lighter in the hand if under the water though.


Ditto, the gold is already displacing water in the pan, so more water in the pan or around the pan make no difference on the gold, but does make a difference on the weight of the pan and water in it compared to the weight out of the water.
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Re: Rookie question

Postby TRJ » Thu Feb 09, 2017 7:08 am

Darth Placer and Gold Seeker are 100% correct.

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Re: Rookie question

Postby Sam Burgin » Sat Feb 11, 2017 5:16 pm

What Steve said.

At rest, the pan and the materials within the pan are in equilibrium. Nothing is going anywhere, unlike a stream with a continuous fluidization. The downward force of gravity and the upward hydrostatic pressure are equalized. Nothing moves.

When the medium is fluidized, the hydrodynamic pressure falls and causes the heaviest particles to displace the lighter elements by volume and mass.


That's why you must sieve various stages of the black sands so the gold will always compete with the particles of equal volume, 360 mesh gold will practically jump out of the pan if everything in the pan is 360 mesh or smaller.

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Re: Rookie question

Postby jdtrailblazer » Fri Feb 17, 2017 7:24 pm

Good post!
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