What is Upthrust: Definition and 35 Discussions

Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. The pressure difference results in a net upward force on the object. The magnitude of the force is proportional to the pressure difference, and (as explained by Archimedes' principle) is equivalent to the weight of the fluid that would otherwise occupy the submerged volume of the object, i.e. the displaced fluid.
For this reason, an object whose average density is greater than that of the fluid in which it is submerged tends to sink. If the object is less dense than the liquid, the force can keep the object afloat. This can occur only in a non-inertial reference frame, which either has a gravitational field or is accelerating due to a force other than gravity defining a "downward" direction.Buoyancy also applies to fluid mixtures, and is the most common driving force of convection currents. In these cases, the mathematical modelling is altered to apply to continuua, but the principles remain the same. Examples of buoyancy driven flows include the spontaneous separation of air and water or oil and water.
The center of buoyancy of an object is the center of gravity of the displaced volume of fluid.

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  1. brotherbobby

    Sinking a toy boat with lead pellets

    Attempt : I begin by plotting the graph of depth x against mass of pellets m as shown alongside. Using ##m_B = \Delta m_L\Rightarrow m_B = \rho_L A_B\, x(B)## where ##\rho_L## is the density of the liquid, ##A_B## is the cross-sectional area of the boat and ##x(B)## is the depth to which the...
  2. B

    Engineering Calculating Upthrust and Tension for a Submerged Pillar

    Hello, Looking for help on the following: A crane is used to lower a vertical cylindrical pillar into a reservoir. The pillar has a mass of 4 tonnes and is submerged to a depth of 2.5m. It has a diameter of 1.1m. The specific gravity of fresh water is 1125kg/m^3, and 9.81m/s^2. Use a free...
  3. L

    Mars Rover Upthrust and Acceleration calculations help 🚀

    1. ρmars atmosphere = 0.02 kgm^-3 volume of a sphere = 4 / 3 𝜋 𝑟^3 gmars = 3.8 ms^-2 So: Volume of a hemisphere=2/3 𝜋 𝑟^3 r=7.75 m Archimedes' principle states that the upthrust on an object is equal to the weight of fluid that the body displaces To find the upthrust produced...
  4. brotherbobby

    Will the displacement of a solid ball affect the water level?

    (The picture below is my drawing. I followed the instructions of the problems and drew for reasons of clarity.) Let me start by writing down the given details : Volume of drum ##V_D = 0.05 m^3##, mass of drum ##m_D = 5 kg##, height of water column (initially) ##h_W = 1 m##, base area of water...
  5. brotherbobby

    Apparent weight of body immersed in liquid

    I have to assume that ##h## is the height of the body. The graph above shows how the apparent weight of the body changes as it is immersed into the liquid. In (a), after immersing the whole height of the body, the apparent weight ##W_{\text{app}} > 0##. Hence the upthrust ##U < w_B \Rightarrow...
  6. jisbon

    Solving for Maximum Speed of Cube: Kinematics & Upthrust

    So, I recognise that: $$ma=pg\left( L^{2}\right) \left( L-y\right) -\dfrac {1}{2}pgL^{3} $$ whereby $$pg\left( L^{2}\right) \left( L-y\right)$$ is the upthrust while the other is mg. So, to find the largest speed of cube, I will assume that acceleration is zero since the acceleration slowly...
  7. jisbon

    Calculating Density Using Upthrust and Drag Force in Water

    So using FBD, I came out with: since it reaches terminal velocity, I can assume F=0, thus mg = upthrust +drag So my density = mg - drag force/g * volume However the answer seems to be wrong from the answer key I was given. Any ideas why? Thanks
  8. J

    B Upthrust & Buoyancy: Does Surface Area Matter?

    Hi, Really quick general question but is upthrust independent of surface area. I came across the following: "Heavy things can float on water if there is sufficient surface area to produce a big enough upthrust at a particular depth" My thinking that this is incorrect is based on the...
  9. WherE mE weeD

    Should I Use the Total Length of an Object to Calculate Buoyancy?

    this is a float submerged in fluid the upthrust is greater than the weight. P=ρgh to calculate the pressure should I use the total length of this object as h? Homework Equations P=ρgh Fb=PA (A=area) The Attempt at a Solution Curious to know whether I should use the total length of this object...
  10. Muhammad asad

    B Upthrust in Liquid: Can Objects Keep Sinking?

    I have a question about upthrust. I have studied that if an object is placed in a fluid and if the upthrust is not equal to the weight of the object even after the object is fully immersed in the fluid, it sinks. My question is that is it possible for an object immersed in water to keep sinking...
  11. ThatOneMidget

    Question about the upthrust of a rocket

    Homework Statement The Saturn V rocket which launched the Apollo space missions had the following specifications: mass at lift-off = 3.0 x 10^6 kg velocity of exhaust gases = 1.0 x 10^4 m/s initial rate of fuel consumption at lift-off = 3.0 x 10^3 kg/s 1.(a) Calculate: (i) the force (thrust)...
  12. J

    Upthrust Reaction - Water Injection @ 5bar -8msw Subsea

    Hello, I have a problem Basically, in theory -- a damaged 1220mm dia pipeline sub-sea (-8msw) with a 500mm hole in the 12oclock position 600mm. Pipeline resting on the seabed The goal of the problem is to flush the pipeline of its contents in both directions. The idea - have a containment...
  13. Kaneki123

    B Upthrust & Floating: Confirm Position in Liquid

    OKay I just want to confirm...It is written everywhere that if , in a liquid, the upthrust acting on an object is equal to its weight, it will float...If we consider an object taken deep into a liquid, and somehow its weight becomes equal to the upthrust acting on it, then it would just stay at...
  14. C0balt

    Upthrust/ how does the weight of a sphere change in water?

    Homework Statement See image attached. Oh it's part c by the way. Homework Equations maybe upthrust=weight of water displaced... None really relevant. The Attempt at a Solution I thought the balance would initially go up as the sphere entered the water, (but maybe slightly less than the...
  15. M

    Upthrust vs Weight: Object Immersion in Fluids

    Homework Statement Object immersed in fluid. If weight larger than upthrust, it sinks. If less, it floats. If weight equal to upthrust, it floats, right? Thanks! Homework Equations N/A The Attempt at a Solution N/A
  16. O

    Is the Upthrust Equation for Fully Submerged Objects Incorrect?

    Please may somebody explain why this equation for the pressure at the bottom of a fully submerged object is wrong (assuming it is a cube). Thank you! Pressure = ( density of the fluid x h[1] x g ) + ( density of the cube x (h[2] - h[1] ) x g)
  17. J

    Origin of Upthrust: Displaced Water & Pressure Difference

    Homework Statement When an object is submerged or floats in a fluid, the.. Pressure on bottom surface > Pressure on top surface So the resultant force is acting upwards. Why the difference in pressure? Must both, the top and bottom surfaces, have the same area of contact with the fluid? Since...
  18. Suraj M

    Solving Buoyancy & Upthrust: Understand C & D

    Homework Statement A solid is completely immersed in a liquid. The force exerted by the liquid on the solid will(more than one right) (a)increase if it is pushed deeper inside the liquid. (b)change if its orientation is changed (c)decrease if it is taken partially out of the liquid (d)be in the...
  19. L

    Measuring liquid density using upthrust

    Hi, I brew beer and part of the process is measuring the specific gravity of the fermenting liquid. This is done visually using a hydrometer. Digital hydrometers are available and very expensive. I'm trying to come up with a way to measure the liquid specific gravity electronically. Typical...
  20. C

    Upthrust: Weight of Object or Fluid?

    I got confused with the Archimedes theory. I want to ask if upthrust equals to the weight of the fluid displaced by the object, or is it the weight of the object itself? I thought it should be weight of fluid since fluid has it's own density which determines it's mass which in turns affect...
  21. A

    What is the cause of upthrust experienced by a body placed in a

    what is the cause of upthrust experienced by a body placed in a liquid.
  22. H

    Upthrust in a fully submerged object

    When we put any object in a fluid it experiences upthrust. Suppose an iron cube is put in a beaker with water. It sinks and reaches bottom. When it reaches bottom and touches the base of beaker there is no water below it. Then will it experience upthrust?
  23. J

    How Do You Calculate Thrust and Pressure in Fluid Dynamics and Aeronautics?

    Homework Statement 1)a water tank has a rectangle base of area 5m x 4 m and is 3 m deep. if it is full of water, calculate the thrust on the base and on each side. 2)a model helicopter of mass 5 kg rises with constant acceleration from rest to a height of 60 m in 10 s. find the thrust exerted...
  24. H

    Object at Beaker Base: Upthrust or Not?

    if an object is at the base of a beaker does it feel upthurst even though there is no liquid below it
  25. S

    Upthrust in water with different gravity

    Homework Statement A small cork floats in water, exactly half submerged, on Earth. If the container, water and cork were all transferred to a place where the acceleration due to gravity is less than that on Earth, would the submerged proportion of the cork be greater. stay the same or...
  26. S

    When Does Upthrust Change?

    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...
  27. S

    Explain when will upthrust change?

    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...
  28. A

    Calculating upthrust on a solid object

    Calculating upthrust on an object Homework Statement An object made entirely of one material has a volume of 30m3. It floats in a liquid of density 30 g/cm3 with only half it's mass below the liquids surface. The object is then placed in a liquid of the same volume but with a density of 1...
  29. L

    Upthrust: Question on Aluminium & Brass Spheres

    Homework Statement 2 small metal spheres of the same size but made of aluminium and brass are released simultaneously from rest inside a tall vessel filled with a viscous liquid.Consider the following statements A) Upthrusts on both spheres are the same B)The initial accelerations of both...
  30. A

    Solving Physics Problems: Gravitation, Upthrust & Buoyancy

    [SOLVED] Physics Probs HI there!:cool: Can u just give some more ideas and explanation on Gravitation,Upthrust, Buoyancy?:confused: Some numericals to explain n to add on would b better.:smile:
  31. N

    Upthrust calculations for hovercraft

    Hi, usually don't ask for help but this is bugging me now! I am working on a r/c model hovercraft, i have been trying to work out the upthrust but can not get a formula to do so. I know the air cushion pressure is a factor, i am using a ducted fan for upthrust with attatched 9 v motor. Any...
  32. C

    Exploring the Differences Between Viscosity and Upthrust in Liquids

    Viscosity and Upthrust... Viscosity and upthrust are both forces which occur in liquids, and must both rely on electrostatic effects. So what is there difference between them? What causes these 2 distinctly different effects? Thanks in advance. :smile:
  33. Z

    Upthrust Question: 2 Conditions for Independence on Depth

    I found this question about upthrust "give two conditions for the upthrust of a body to be independent upon the depth of imersion" Any ideas?
  34. P

    Is There Upthrust on a Smooth Object on a Smooth Ocean Floor?

    would there be an upthrust or not if a perfectly smooth object was on a perfectly smooth ocean floor? I expect that since there is no water beneath the object there is no pressure to provide an upthrust, there would only be pressure above the object creating a downward force!?
  35. J

    Upthrust Problem: Exploring Forces & Balance Readings

    I would like to discuss the following problem which is about upthrust. Please give me corrections if there is anything I did wrongly. Question: The solid metal ball is suspended from the spring balnace S as shown so that it is just above the surface of the water in the displacement can. The...