What Does My Finger Weigh?

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• posted on September 4, 2016, 8:32 pm
I'm always interested in learning new things to try and spotted this one in a book of puzzles SWIMBO just gave me. I just had to try it.
Put a paper or Styrofoam cup with water in it on a kitchen or postal scale and note its weight. Leave it on the scale. (A glass of water might be too heavy for the scale.)
Now stick a finger or two into the water without touching the side or bottom of the cup and watch what happens.
The scale reading increases.
Did you expect that? I didn't.
I'm sure any physicists here will explain why.
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)

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• posted on September 4, 2016, 10:03 pm
Jeff Wisnia wrote:

I'm not a physicist, but it sounds like the bottom of your finger had been exerting more vertical force (in the downward direction) on the "surface" of the water, than the air had. There is also Newton's 3rd Law (which I just looked up), “For every action, there is an equal and opposite reaction.” OTOH, I wouldn't have expected to see the phenomenon you described either! ; )
Bill

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• posted on September 4, 2016, 10:16 pm
"Jeff Wisnia"

Easy. You added the mass of your finger below waterline to the mass of water already in the cup.

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• posted on September 4, 2016, 11:27 pm
On 9/4/2016 6:16 PM, Phil Kangas wrote:

How? Unless it is only partially filled, but if filled, no. If he suspended his finger it would displace the water. The spilled water would lower the weight and be replaced by the weight of the finger . Looks like he left out a detail.

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• posted on September 5, 2016, 12:58 am
"Ed Pawlowski" <

Jeff did not say the cup was full, only that it had some water in it.

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• posted on September 5, 2016, 12:32 am
Phil Kangas expressed precisely :

It's not the mass, it's the displacement of a volume of water.
Eureka!!

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• posted on September 4, 2016, 10:24 pm

Depends. How many boogers on finger in question?

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• posted on September 6, 2016, 9:16 pm
notbob posted for all of us...

Not any on my fingers because I eat them. Is there anything wrong with that?
--
Tekkie

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• posted on September 4, 2016, 11:20 pm
On 9/4/2016 4:32 PM, Jeff Wisnia wrote:

Eureka ;)

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• posted on September 5, 2016, 12:33 am
Frank explained :

:)
Will anyone look that up I wonder?

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• posted on September 5, 2016, 12:36 am
On 9/4/2016 5:33 PM, FromTheRafters wrote:

Buoyancy. You look it up.

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• posted on September 5, 2016, 1:37 am
Taxed and Spent formulated on Sunday :

No shit, Sherlock.
https://en.wikipedia.org/wiki/Eureka_ (word)#archimedes

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• posted on September 5, 2016, 12:00 am
On Sunday, September 4, 2016 at 3:32:53 PM UTC-5, Jeff Wisnia wrote:

My guess is the surface tension broken by the finger exerts a downward force.

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• posted on September 5, 2016, 2:30 am
On Sun, 4 Sep 2016 16:32:44 -0400, Jeff Wisnia

You are exerting a force equal to the bouyancy of your finger on the cup.

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• posted on September 5, 2016, 3:59 am
wrote:

Yup. Imagine this: put your finger into the water and mark it at waterline. Cut it off at the line and put it back into the cup. If the specific gravity is higher than water it will sink to the bottom adding weight to the scale minus the bouyancy of it. If the specific gravity is less than water then the cutoff finger stub will float. By attaching a thin wire you can push it into the water till it is flush. This applied force will add to the scale by the ratio of sp.gr. of water vs. finger stub. By comparing the scale numbers you could calculate the specific gravity of your finger! If the test object is heavier than water and you suspend it by a thin wire the scale indication will not change.The tension in the supporting wire will decrease due to bouyancy. If the wire was attached to a scale you could use that to calculate the sp.gr. of the object of interest. hth phil k.

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• posted on September 6, 2016, 1:58 am
message wrote:

Gotta add this comment to my explanation. The free floating finger chunk will add to the scale the weight of water displaced by it. Pushing it in so that the mark is at waterline adds a little bit more to the scale reading. Just like when it was still attached.

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• posted on September 5, 2016, 3:45 am
On Sun, 4 Sep 2016 16:32:44 -0400, Jeff Wisnia

You really just weighed the amount of water displaced but since the specific gravity of a finger is pretty close to 1 it is a good approximation. If you did it with a lead sinker, you result would be different.

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• posted on September 5, 2016, 1:09 pm
On 5/09/16 04:32, Jeff Wisnia wrote:

> I'm sure any physicists here will explain why.
You talking about Archimedes's Principle? https://en.wikipedia.org/wiki/Archimedes%27_principle
BUT, a finger is made up of bones and flesh, of different density!
--
@~@ Remain silent! Drink, Blink, Stretch! Live long and prosper!!
/ v \ Simplicity is Beauty!

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• posted on September 5, 2016, 1:28 pm
On 9/5/2016 9:09 AM, Mr. Man-wai Chang wrote:

So? It will still displace an amount of water equal to the volume of the finger. A 1" cube displaces 1 cubic inch be it hollow plastic or solit lead. The buoyancy will, of course, change.

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• posted on September 5, 2016, 2:09 pm
On 5/09/16 21:28, Ed Pawlowski wrote:

Read the subject.. the "Finger Weigh" part... :)
--
@~@ Remain silent! Drink, Blink, Stretch! Live long and prosper!!
/ v \ Simplicity is Beauty!

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