What Does My Finger Weigh?

[Jeff Wisnia]

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

[Bill]

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

[Phil Kangas]

"Jeff Wisnia"

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

[notbob]

Depends. How many boogers on finger in question?

[Frank]

Eureka ;)

[Ed Pawlowski]

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.

[bob_villa]

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

[FromTheRafters]

Phil Kangas expressed precisely :

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

Eureka!!

[FromTheRafters]

:)

Will anyone look that up I wonder?

[Taxed and Spent]

Buoyancy. You look it up.

[Phil Kangas]

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

[FromTheRafters]

No shit, Sherlock.

[clare]

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

[gfretwell]

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.

[Phil Kangas]

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.

[Mr. Man-wai Chang]

You talking about Archimedes's Principle?

BUT, a finger is made up of bones and flesh, of different density!

[Ed Pawlowski]

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.

[Mr. Man-wai Chang]

Read the subject.. the "Finger Weigh" part... :)

[Mr. Man-wai Chang]

BTW, all previous replies didn't mention Mr. Archimedes. Weird?

Because they all hated *ancient* Greeks? Islamic? :)

[FromTheRafters]

Two of us said "Eureka" and I even posted a URL with Archimedes in the string.