TK .. as a control experiment to your last vid of the ITE what happens when you don't use the vessel cap [I know it had a breather] ?

If you leave it open & completely submerge the Pod to just below the surface & attach it to an external stand it will show a reading on the scale.

If you lower it further to near the bottom what does the scale show while you are lowering it by hand & again when it is clamped to the external stand ?

What does it read if you lower it gradually, perhaps using some sort of screw or whatever you have available etc ?

Quote from: fletcher on September 14, 2012, 02:42:57 AM
TK .. as a control experiment to your last vid of the ITE what happens when you don't use the vessel cap [I know it had a breather] ?

If you leave it open & completely submerge the Pod to just below the surface & attach it to an external stand it will show a reading on the scale.

If you lower it further to near the bottom what does the scale show while you are lowering it by hand & again when it is clamped to the external stand ?

What does it read if you lower it gradually, perhaps using some sort of screw or whatever you have available etc ?

Well, I can do that, but once the thing is fully submerged, since it's fairly rigid its volume won't change with increasing water pressure due to depth. So the amount of water displaced won't change, except by the volume of more pushrod going into the water, negligible I hope. At least that's what I expect. Therefore the buoyant force won't change after full submergence. If there is volume change allowed, like a Cartesian diver, then that's different, but still, I believe, linear force-pressure relationship.

Unfortunately I don't have any labjacks here. I am guessing that raising the reservoir up and having the Pod fixed is equivalent enough, that might be an easier way to do it.

So how about this. My floater displaces 221 mL and weighs 25 grams, so when free to float should have an upward buoyancy of 221-25=196 grams, or when restrained rigidly underwater the full 221 grams, I think. So if I put the reservoir on the scale, and have a means of raising this up on a jack, and then have the pod attached to the Mark-10 force gauge facing down, once the pod is completely submerged, the scale weight of the chamber+water should increase by 221 grams and remain there as the relative depth increases, and the Mark-10 reading the buoyancy force directly should read the same thing.  The deepest tank I have available is the big glass jar used in my underwater wireless experiments and the max capacity of my scale is 5 kg. I think I can set this up with my kit. The only thing missing is a lab jack. I may be able to improvise one or use a hydraulic jack, or just try to lower the pod smoothly with force gauge instead.

So my prediction is that the force will remain constant as the depth is increased once the thing is fully submerged. When it's clamped to the external stand, that is, what I call external restraint, you will see the buoyancy force added to the weight of the water and reservoir. As I lower by hand, ditto.

But what are we showing or testing by this experiment?

So, blithely assuming Newton still holds, I dispensed with the fancifications and went ahead and did Fletcher's control experiment. The force upward is equal to the force downward, I hope we can agree on that much without actually having to measure it. So just measuring the weight of the reservoir and the water, as the Floater is lowered into the water, should be enough information, I hope.

http://www.youtube.com/watch?v=_aqE9A_0WRg

I thought that was the basis of your ITE video - perhaps I misunderstood ? - i.e. that when fully submerged near the surface the floater/pod displaced an amount of water - when this was 'connected' to the vessel cap so the pod couldn't rise further, but was still submerged, the entire setup was one unit & weighed so much on the scale.

When you depressed the pod slowly [you now held the weight of the pod] the scale read higher by about 200 gms even though the depth of fluid hardly changed [just some small displacement volume & buoyancy from the shaft perhaps] - as long as you held the end of the pod shaft, acting as an external support, the scale continued to read higher, without any apparent water depth increase ?

AYK the pressure force of water with depth acts in all directions - therefore it acts on the bottom of the vessel [internal force] & also pushes up on the bottom of the pod just above it as buoyancy.

The control might show that this increase in 'weightforce' reading on the scale only exists while you hold the shaft or it is externally clamped - IOW's it is no longer part of the unit by being directly connected to it - the clamp holds its weight but its displacement causes a force downwards on the bottom of the vessel & an internal force pushes up on the pod.

The control is to then attach the shaft to the vessel when the pod is near bottom so that the vessel holds its weight, instead of externally by you or a separate support - action [work] & equal reaction.

I'll download your video again over the weekend & have another look.

Here is a web site with a problem that everybody should work thru - how to bust the bottom out of a glass bottle & why it does.

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Hyperphysics>Fluids>Pascal's Principles - you should assign a height & volume to the bottle to work thru the results.

TK .. Action & Equal Opposite Reaction - when you depress the floater, water viscosity resists its direction of motion, as the water is forced out of the way => gives higher reading on scale just coincidentally similar to displacement, IMO.