
#1
Aug910, 08:45 AM

P: 18

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?




#2
Aug910, 09:31 AM

P: 124

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 halfsubmerged position. 



#3
Aug910, 09:54 AM

P: 18

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
Aug910, 10:00 AM

P: 124

Upthrust 



#5
Aug910, 10:14 AM

P: 18

Ok nvm, here is the qn: a ship with uniform crosssectional 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
Aug910, 10:23 AM

P: 124





#7
Aug910, 10:33 AM

P: 18





#8
Aug910, 10:42 AM

P: 124

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
Aug910, 10:54 AM

P: 18





#10
Aug910, 10:57 AM

P: 124




Register to reply 
Related Discussions  
explain when will upthrust change?  General Physics  4  
Question on upthrust  Introductory Physics Homework  4  
Viscosity and Upthrust....  General Physics  3  
Upthrust question  Introductory Physics Homework  3  
Upthrust problem  Introductory Physics Homework  1 