I Weight and the Archimedes Principle

1. Feb 3, 2017

gloo

I was wondering - if you had a container of water and you push a hollow object that is buoyant to the bottom so that it is completely immersed - the whole container will be heavier because of water being displaced up in the container.

Now if you release your hand or whatever you use to hold the object down near the bottom -- the object will float up. But is the container still heavier in the time interval that the object is still completely immersed? I am assuming yes because the amount of water displaced up is still the same and the hollow object is not attached to the container but freely moving up in the water.

2. Feb 3, 2017

phinds

Uh ... it will? What is adding the weight? What would happen if you STARTED with a weightless hollow object tethered to the bottom of a weightless container and then filled the container with 10lbs of water that went up over the top of the object. What would the container weigh? In other words, I ask again,what is it that adds weight in the first case? Is any weight added in my scenario? Why or why not?

3. Feb 3, 2017

gloo

If tethered to the bottom it will weight the same since it is now part of the system. But my scenario is that it is not attached to the container but in the midst of floating to the top - but still submerged. The submerged object will still have displaced a volume of water up.

4. Feb 3, 2017

phinds

You have not answered my question.

5. Feb 3, 2017

gloo

Ignoring the weight of the container it would weight less than 10 pounds I am assuming because the water spilled over the top and does not weigh down on the container anymore.

6. Feb 3, 2017

gloo

Sorry, I thought you said it would flow out of the container. Ignoring the weight of the container it would be 10 pounds - the weight of the water because the object that is buoyant is tetthered to the bottom

7. Feb 3, 2017

phinds

You are NOT answering my question. WHAT CAUSES THE EXTRA WEIGHT in your scenario?

8. Feb 3, 2017

gloo

1. In your case with the teeter - no weight is added
2. the weight added in the first place (my scenario) - was my force pushing the hollow object down which then displaces the water higher. My force is causing the water to displace up
3. No weight is added in your scenario because you tethered the hollow object to the container and is now part of the container system

9. Feb 3, 2017

jbriggs444

Back up. Before you start arguing about what causes the extra weight you should define what you mean by the word "weight".

The way the rest of us use it, it is the downward force of gravity on an object and NOT the upward force required to hold an object in position against a sales clerk with his thumb on the scale.

http://www.saturdayeveningpost.com/...nd-the-canvas-art-entertainment/rockwell.html

10. Feb 3, 2017

phinds

And what happens when you stop pushing down?

11. Feb 3, 2017

gloo

the object will start to move up to the surface

12. Feb 3, 2017

phinds

What happens on the SCALE ? That's what your original question was about

13. Feb 3, 2017

gloo

The scale will show less weight once the object is floating on top of the surface

14. Feb 3, 2017

phinds

What I have been trying to help you see is that it is not the weight of the displaced water that causes the scale to show a larger value when you press a lighter-than-water object into it, it is the force you have added by pushing the weight. When you release the weight you are no longer adding any force and so the system now acts exactly as it would if the object were tethered to the water level. It becomes irrelevant whether the object is under the water or floating on top of it.

15. Feb 3, 2017

Staff: Mentor

16. Feb 3, 2017

Staff: Mentor

Can we all just get along? Thread reopened.

17. Feb 3, 2017

jbriggs444

Buoyancy is all about hydrostatics -- equilibrium situations. The questioner here is asking about a transient state in which the system is far from equilibrium. The answer is unlikely to be obvious. [or even unambiguous]

18. Feb 5, 2017

NTL2009

So is my thinking straight on this aspect of the question -

We place a shallow pan on a scale, then add one liter of water. We expect the scale reading to increase by ~ 1 kg (within the accuracy of the scale, our volume measurement, purity of water, temperature, etc).

We remove that container, then place a very tall container on the scale, then add one liter of water. I think we could expect the scale to read a bit more than 1 kg weight (assuming the scale would have enough sensitivity/resolution). The water in a tall column provides more downward force, just like the water in a municipal water tower. Correct?

The mass has not changed, only our measured weights.

edit - wait, the pressure/area is higher, but the area is less. So no change in weight?

Last edited: Feb 5, 2017
19. Feb 5, 2017

Staff: Mentor

No change in weight. :)

20. Feb 5, 2017

phinds

You are confusing water pressure w/ weight. As Drakkith said, no change in weight

21. Feb 5, 2017

NTL2009

Yes, thanks, I see that now (and was pretty sure I figured it out just after I posted - see my edit).

22. Feb 6, 2017

sophiecentaur

There will be small transitional forces on the scale during the process. Ignore the force involved in removing your finger (perhaps use an infinitely thin rod to push it down). When you first release the object, the recorded weight will be Weight of Bowl, water and object. The object will then accelerate upwards until it reaches a terminal velocity. That will temporarily increase the force on the scale. Likewise, when the object reaches the top, it will slow down and the recorded weight will dip for a short while until the total weight is Bowl, water and object again. Depending upon the damping coefficient, there could be a number of cycles of variation of recorded weight whilst the object 'bobs' up and down a bit.

23. Feb 6, 2017

jbriggs444

I believe that you have this backward. The object accelerates upward only because the water is accelerating downward. The net effect is to temporarily reduce the force on the scale.

Agreed, and I think this is the important part. There will be oscillations.

24. Feb 6, 2017

sophiecentaur

Ummm. Yes. I think you are right to challenge it. There is a greater mass of water accelerating downwards than of object rising so perhaps the 'weight would appear to be less'. If it were a bubble, the rising mass would be negligible so it would be the equivalent of the whole mass of water falling to fill the space. Question is whether this reduction applies as the object (/bubble) continues rising or is it just at the start? I think it must apply just at the start because acceleration stops fairly quickly.
Any more ideas, anyone?