When does upthrust change

In summary: However, the weight of the displaced fluid will be different and therefore the upthrust will be different. In summary, the positioning of objects does not affect the magnitude of upthrust, only the volume of fluid displaced and density of fluid displaced. When a ship is floating, the upthrust will be the same regardless of the density of the liquid. However, if the ship is completely submerged, the upthrust will depend on the density of the liquid.
  • #1
starwars32
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Hi.. Can someone explain when will upthrust change? cos i come across books saying that upthrust will not change when two objects of different density but same material are placed in the same fluid. I also come across that upthrust will not change when same objects are being transferred from liquid a of different density to liquid b of another density, the upthrust also will remain same. Can some one pls explain to me when and when will upthrust not change? Under wad conditions thanks! I thought that upthrust is dependent on density of liquid? as U= Vrollg?
 
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  • #2


upthrust is calculated using the archimedes principle. the magnitude of upthrust is the same as the weight of the fluid displaced.

so of course it depends on the volume of fluid displaced (and hence the volume of the object that displaces the fluid) and density of fluid displaced. Note that the density, shape, mass or density of the object does not affect the magnitude of upthrust. Only the volume of the immersed object will affect the magnitude of upthrust. So a styrofoam cube and a metal cube of the same volume will experience the same upthrust, but of course the metal cube has a greater weight than the upthrust and the net force is downwards and it sinks. The styrofoam cube has a smaller weight and therefore it will only sink to the extent that the upthrust (= weight of the fluid displaced) = its own weight, and it will just float at this half-submerged position.
 
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  • #3


so the positioning of the objects does not determine the magnitude of the upthrust ? cos different density?
Next qn: a lot of people say that under different fluids, the upthrust will change. But wad about for the same object, at first u put it in fresh water of density 1, and then u shift it to sea water of density 2, the upthrust will change?
 
  • #4


starwars32 said:
so the positioning of the objects does not determine the magnitude of the upthrust ? cos different density?

Yes positioning doesn't affect upthrust, as long as the submerged volume is the same. The positioning only affects the fluid resistance/drag force as the object moves through the water, which isn't relevant here.

Next qn: a lot of people say that under different fluids, the upthrust will change. But wad about for the same object, at first u put it in fresh water of density 1, and then u shift it to sea water of density 2, the upthrust will change?

Yes. Assuming that the volume of the displaced fluid is the same, if the density doubles, the weight of the displaced fluid also doubles and therefore the upthrust doubles as well. Upthrust is proportional to the density of the fluid.
 
  • #5


Ok nvm, here is the qn: a ship with uniform cross-sectional area A is floating in the sea. THe ship then unloads its cargo and rise a distance y. Given the density of sea water is Psea water, find the weight of the cargo W in terms of Pseawater, A, Y and g where g is the acceleration due to gravity.
ANs: AgyP seawater

Another identical ship carrying the same cargo is floating in fresh water. The ship then unloads its cargo and rises a distance z. Given that the density of fresh water is Pfresh water, find the weight of the cargo W in terms of Pfresh water, A, z and g.
Ans: AgzP freswater

Hence find the ratio of Pseawater/Pfreshwater in terms of y and z.
Ans : Pseawater/ Pfreshwater= z/y

Okay here is the gist of the qn: Suggest whether the upthrust in the fully laden ship will change when the ship moves from fresh water to sea water.
Ans given was no.
 
  • #6


starwars32 said:
Ok nvm, here is the qn: a ship with uniform cross-sectional area A is floating in the sea. THe ship then unloads its cargo and rise a distance y. Given the density of sea water is Psea water, find the weight of the cargo W in terms of Pseawater, A, Y and g where g is the acceleration due to gravity.
ANs: AgyP seawater

Another identical ship carrying the same cargo is floating in fresh water. The ship then unloads its cargo and rises a distance z. Given that the density of fresh water is Pfresh water, find the weight of the cargo W in terms of Pfresh water, A, z and g.
Ans: AgzP freswater

Hence find the ratio of Pseawater/Pfreshwater in terms of y and z.
Ans : Pseawater/ Pfreshwater= z/y

Okay here is the gist of the qn: Suggest whether the upthrust in the fully laden ship will change when the ship moves from fresh water to sea water.
Ans given was no.

Of course no. If a ship is floating that means the upthrust = weight of the ship such that there is no net force acting on the ship and it is in mechanical equilibrium. And since the fully laden ship has the same weight whether it is in the freshwater or seawater, the upthrust is the same.
 
  • #7


arkofnoah said:
Of course no. If a ship is floating that means the upthrust = weight of the ship such that there is no net force acting on the ship and it is in mechanical equilibrium. And since the fully laden ship has the same weight whether it is in the freshwater or seawater, the upthrust is the same.

Hmm..so if its floating then upthrust will be same if different density of liquid but when its submerged, then upthrust will not be same if different density?
 
  • #8


starwars32 said:
Hmm..so if its floating then upthrust will be same if different density of liquid but when its submerged, then upthrust will not be same if different density?

Yes that's right. The ship will displace different volume of water in seawater and in freshwater. The sink will sink a bit more in freshwater than in seawater because freshwater is less dense and more water needs to be displaced to support the ship's weight, but the upthrust will end up to be the same in both cases.

If the ship is completely submerged then the volume of water displaced is the same, so now the density will affect the weight of the water displaced and therefore the magnitude of the upthrust.
 
  • #9


arkofnoah said:
Yes that's right. The ship will displace different volume of water in seawater and in freshwater. The sink will sink a bit more in freshwater than in seawater because freshwater is less dense and more water needs to be displaced to support the ship's weight, but the upthrust will end up to be the same in both cases.

If the ship is completely submerged then the volume of water displaced is the same, so now the density will affect the weight of the water displaced and therefore the magnitude of the upthrust.

so we do apply archimedes law here and principle of floatation here?
 
  • #10


starwars32 said:
so we do apply archimedes law here and principle of floatation here?

For your question? Neither. Just Newton's first law which states when an object is at rest, the net force is zero and therefore upthrust = weight.
 

1. When does upthrust change in a liquid?

Upthrust changes in a liquid when the density of the liquid or the object placed in it changes.

2. What factors affect the change in upthrust?

The density of the liquid and the object, as well as the volume of the object, all affect the change in upthrust.

3. How does the shape of an object affect upthrust?

The shape of an object can affect upthrust because it determines the amount of volume displaced in the liquid, which in turn affects the upthrust force.

4. Does the depth of the liquid affect upthrust?

Yes, the depth of the liquid can affect upthrust as it changes the pressure and density of the liquid, which in turn affects the upthrust force.

5. Can upthrust ever be greater than the weight of an object?

Yes, upthrust can be greater than the weight of an object if the object is less dense than the liquid it is placed in.

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