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How is the strain gauge set up? If it's all within the shadow of the cube it probably won't register anything either way.
Bob at PC
-----Original Message-----> >and simplify the analysis by not paying attention to the
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Chuck Britton
Sent: Monday, November 08, 2010 9:04 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] definitions ... purely operational, or not
So the strain gauge measurement does or does NOT differ with water
under the block or not under the block?
At 1:51 PM -0700 11/7/10, John Denker wrote:
>On 11/07/2010 07:35 AM, Chuck Britton wrote:
>
>> I have tried to avoid such definitional difficulties by specifying
at
>> strain-gauge style force measurement on the bottom of the aquarium.
>
>Works for me.
>
>> Does the flexing of the bottom depend on whether a layer of water
>> exists between the lead cube and the bottom of the aquarium?
>
>Just to be clear (since a number of variants have been mentioned)
>we are comparing two cases:
> a) The block has some narrow feet, such that there is water
> *almost everywhere* between the block and the thing it is
> resting on. The feet occupy a negligible percentage of the
> area, but are needed to support the net weight of the block.
> Here "net weight" means the total dry weight minus the buoyancy.
> b) The block has no feet, and conforms to the bottom of the
> aquarium, so that there is water *almost nowhere* between
> the block and the thing it is resting on.
>
>> I think
>> not, but others seem to differ in there opinions? or not.
>
>There is no difference in the total force between case (a) and
>case (b). There are profound force-balance arguments and
>conservation of momentum arguments that require it.
>
>There may be subtle differences in how the force is distributed,
>but these can be minimized by artful engineering. Fluids
automatically
>distribute the force evenly, whereas solid/solid contacts might not.
>
>> I'm also trying to avoid the need for 'suction-cup effects' in
>>this analysis.
>
>You can't avoid it. Not in case (b) anyway.
>
>Think about it in terms of the principle of virtual work. If
>the notion of "force" is to mean anything at all, we need to
>consider what happens if we raise the block an infinitesimal
>amount. Thereupon either an infinitesimal amount of water
>rushes into the gap underneath the block ... or it doesn't.
>The force that you get is wildly sensitive to this.
>
>I've said it about five times now: This issue is absolutely
>central to the entire discussion. You can get any answer you
>want, positive or negative, depending on whether you assume
>the water can weasel in or not. People will continue talking
>past each other and going around in circles until they deal
>with this issue.
>
>As I said in my very first note on the topic, the case of a
>block "just barely" conforming to the shape of the bottom is
>a pathological case. More formally, it is a singular limit.
>The water pressure is present for all h>0 and absent for all
>h<0. And vice-versa for the solid/solid contact force. The
>sum of the two is constant, but if you insist on asking about
>the two forces separately, there is going to be a discontinuity.
>
>Some clever person (CB) suggested using a trampoline to soften
>the discontinuity, but that doesn't entirely solve the problem.
>At some point it becomes a dynamics problem: How fast can the
>water enter/exit the gap?
>
>If you insist on a simple model of contact versus non-contact,
>discontinuities and/or the dynamics ... then by this point the_______________________________________________
>bathwater and the baby are long gone. The water that rushes
>into that tiny gap (or not) is central to any semblance of
>understanding of this system.
>_______________________________________________
>Forum for Physics Educators
>Phys-l@carnot.physics.buffalo.edu
>https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
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