What Happens to an Object in a Vacuum Filled Jar?

  • Context: Undergrad 
  • Thread starter Thread starter LeoYard
  • Start date Start date
Click For Summary

Discussion Overview

The discussion revolves around the behavior of a floating object in a jar partially filled with water when air is evacuated from the jar using a vacuum pump. Participants explore the implications of reduced air pressure on the object's buoyancy and the properties of water in a vacuum environment.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant suggests that if a vacuum is created, the water may turn into vapor, which would affect the object's buoyancy.
  • Another participant conjectures that the density of the water would decrease, potentially causing the object to sink.
  • A different viewpoint questions the assumption that the object's density remains unchanged, particularly if the object is not rigid.
  • Some participants argue that if the object is rigid and not elastic, it would retain its density, which is greater than that of the expanded liquid.
  • Another participant posits that typical objects would continue to float since water is not very compressible, suggesting that the water level would rise slightly.
  • One participant revises their earlier thought, indicating that the amount of water and the size of the container may influence whether the object floats on the liquid surface or not.

Areas of Agreement / Disagreement

Participants express differing views on how the object will behave in a vacuum, with no consensus reached on the outcome. Some believe the object will sink, while others argue it will continue to float, depending on various factors.

Contextual Notes

Participants acknowledge assumptions about the object's density and rigidity, as well as the compressibility of water, which remain unresolved in the discussion.

LeoYard
Messages
16
Reaction score
0
A jar filled partially with water has an object floating in it. The jar is open. Now, if the jar is closed and a vacuum pump is used to pump the air out, what will happen to the floating object? will it rise up more or sink or ...? and why?

What do you think of the following approach :

Normally, It would be impossible to pump all the air out but if you theoretically did, (assuming that the jar doesn't shrink) you would be forcing the water to occupy a large space thus turning it into water vapour. (similarly, if you add pressure to a tank of propane (vapour), it turns into liquid due to pressure.

So if it turns into gas, the object will no longer float.

But thinking more logically, If the object floats in water, that would mean that there is either air trapped in the object or the object is less dense than the water.

If there is air, the vacuum would remove it, thus expanding (more space between the particles) the object at the same rate as the water.

So assuming that the object remains intact (not expanded to the point that the molecules separate), the object will stay in the same position and neither rise or fall.
 
Physics news on Phys.org
If you sucked the air out of the jar, then the density of the water would decrease. Thus the object would be more dense than the water. I conjecture that the object would sink. (I'm not saying I'm right; that's why it's a conjecture!)
 
Cristo, you assume the object will not change in density (and that its density was very close to that of water, to begin with). If it were a marshmallow..
 
cesiumfrog said:
Cristo, you assume the object will not change in density (and that its density was very close to that of water, to begin with). If it were a marshmallow..

That's a very good point! I just went with my gut on that one, which I probably shouldn't do in future! :blushing: However, if the object was pretty much rigid, and was only just floating, then my guess wasn't far off!
 
I'm with Cristo on this one. The OP didn't specify that the floating object was elastic. If it isn't, and there's no reason to think that it is, then it will retain its original density which will be greater than that of the expanded liquid.
 
I reckon the typical object will continue floating, since water isn't very compressible. (Of course, the water level will rise slightly, and I'd naively expect the object to also rise slightly, just less than the water level.)
 
Last edited:
I didn't think of this earlier, but I now believe that it would depend upon how much water there is and how large the container is. The first time around, I was thinking that all of the liquid would expand to fill the available volume. Now I'm inclined to believe that there'll be liquid on the bottom with vapour above. If that's the case, then the object will float on the liquid surface.
 

Similar threads

Undergrad Pressure: Sealed bottle physics
  • · Replies 29 ·
Replies
29
Views
5K
Undergrad What happens to the buoyant force at the surface of the water?
  • · Replies 11 ·
Replies
11
Views
2K
Undergrad Do refrigerants passing through a venturi act like steam?
  • · Replies 8 ·
Replies
8
Views
3K
High School Can Vacuum Pockets Make Objects Float?
  • · Replies 13 ·
Replies
13
Views
5K
Does Throwing an Object Overboard Affect Pond Water Level?
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Pumping liquid around and draining a part of it, how much is lost?
  • · Replies 3 ·
Replies
3
Views
667
Undergrad What is the pressure of trapped air inside this tube?
  • · Replies 19 ·
Replies
19
Views
1K
Problem with my Venturi operated Drum Vac
  • · Replies 11 ·
Replies
11
Views
3K
Undergrad Thermal Tumbler and unusual noise....
  • · Replies 3 ·
Replies
3
Views
3K
Undergrad Vibration in vacuum. What would happen?
  • · Replies 5 ·
Replies
5
Views
11K