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Re: [Phys-l] Finishing up with that Dead Horse.



On 11/6/2010 10:36 AM, Chuck Britton wrote:
The flexing of the aquarium bottom can be used to quantify the
'weight' of an object placed on the bottom. This procedure can be
used whether the aquarium is filled to the brim with water or is
empty.

A lead cube will 'weigh' less if the aquarium is filled to the brim
with water than if the aquarium were empty.

It will weigh less by an amount equal to the weight of the displaced fluid.

A spring scale could be (often is) used to test this weight reduction
also - but something called the 'suction cup effect' might complicate
things when the cube is on the bottom. I want to avoid the
possibility of any such 'suction cup effect'.

Is my weight reduction hypothesis correct?

(Full to the brim - so , yes, that weight of water WAS removed from the system.

Four concrete cases:
1) 1 cc of lead is suspended on a fine thread in air
lead scale shows 11.3 gm (using lead density = 11.3 gm/cc)
Scale under brimming full aquarium reads x gm
2) 1 cc of lead is suspended at mid water depth
lead scale reads 10.3 gm ( using water density of 1 gm/cc)
scale under aquarium reads x gm)
3) 1 cc of lead rests on sandy bottom of aquarium.
lead scale reads 0 gm
Aquarium scale reads x + 10.3 gm
4) edges of 1 cc cube of lead are sealed to the aquarium floor excluding all water beneath.
Lead scale reads 0 gm
Aquarium scale reads x + 10.3 gm

Floor Plate stress distributions:
Case 1&2) uniform area loading.
Case 3&4) increased area loading under cube, area loading elsewhere unchanged .

(How am I doing?)
Brian W