What is Buoyancy force: Definition and 53 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. Lotto

    How to calculate the velocity of water streaming into the boat?

    The problem: Its solution: I am sure that the velocity ##v## can be calculated by using Bernoulli's equation, but I fail to calculate it. I don't know what pressure is at the hole - is it ##h_2 \rho g## or ##(h_2 - h_1) \rho g##? The pressure of the water "under" the hole is ##h_2 \rho g##...
  2. Lotto

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  3. J

    Calculate the bouyancy of two spheres joined by a rope submerged in seawater

    m * g = mAl * g V * ρ * g = VAl * ρAl * g V * ρ * g = V * ρAl * g ρ = ρAl this does not work at all, because the upper ball must have a density smaller than that of seawater 1200kg/m3 or not?
  4. C

    Exploring the Relationship between Buoyancy Force and Gravitational Force

    Picture below. Both bodies are made of same material but the body placed in water had 2.4 times greater mass. What is the relationship between buoyancy Force and gravitational force between body in water and body in air?I have no clue how to solve this. I know that body with greater mass has 2.4...
  5. MatinSAR

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  6. M

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  7. M

    Work done on moving a submerged object

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  8. Richard Spiteri

    B Exploring Buoyancy: Investigating External Forces and Neutral Buoyancy in Water

    I am still intrigued by the neutral buoyancy of a body and have come up with some other questions. As previously encouraged, I attempted to answer my own questions (this is not home work, it is just me trying to wake up my brain in my retirement). Fig 1 has no question but builds on what...
  9. K

    Iron block dropped into a container of water sitting on a scale

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  10. Richard Spiteri

    I What are the downward forces on a neutrally buoyant body in water?

    The attachment below describes a tank, hollow pipe and two flexible (balloon like) bags forming one body of weight W_tank. My two questions are what are the downward forces acting on the submerged body in both cases.
  11. Hamiltonian

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  12. K

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  13. Navin

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    I feel there is a really obvious flaw in my logic but i can't pin it down So i have to find the thrust on the lamina which is basically force of bauyancy F(bouyancy)= Vρg Now volume of the triangular lamina is its rea into its hieght. v = Ah hence F = Ahρgsome information i feel i didnt take...
  14. L

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    How much more will the volume of a fishingboat go under water, if I load the boat with 3.0m^3 fish with the density 0.90kg/dm^3? Fish : 3.0m^3 Density of fish: 0.90kg/dm^3? Homework Equations Archimedes principle: density * volume * g[/B]The Attempt at a Solution Tried setting upward force...
  15. M

    I calculating the buoyancy force

    Homework Statement A ball with a radius of 1.22 cm is dropped in a mixed drink (specific gravity = 0.910) so that it is submerged. What is the magnitude of the buoyant force acting on the ball? Homework Equations FB=Pfluid*g*Vsubmerged[/B]The Attempt at a Solution I am confused about what...
  16. YoungPhysicist

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  17. chemisthypnos

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    Imagine that there are two metal spheres both with the same volumes and I am trying to get them to float up into the air. I fill the first sphere with 5 ATM of helium and I fill the second sphere with 6 ATM of helium. Will either sphere have a higher buoyancy force acting on it than the other...
  18. K

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  19. P

    Is Newton's 3rd law sufficient to explain buoyancy force?

    I assigned a question: Explain the physics mechanism that results in the buoyancy force. Some students replied: The floating object is interacting with the water. Therefore it exerts a downward force on the water. The water then exerts an upward force on the object due to Newton's Third Law...
  20. Superman123

    Is the buoyant force proportional to the mass?

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  21. 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...
  22. Z

    Yes, that makes sense. Thank you for pointing that out.

    Homework Statement - [/B] What is the buoyant force that acts on a fully submerged 1-L bottle of water? What is the buoyant force that acts on it if it is full of air, not water, but is still fully submerged? Assume you'd have to hold it under water.Homework Equations - [/B]Fb= V x D x GThe...
  23. T

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  24. C

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  25. Ian Baughman

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  26. Ravi Singh choudhary

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  27. N

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  28. vincekillics

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  29. Poquinha

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  30. Poquinha

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  31. B

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  32. S

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  33. U

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    A object B is place on top object B and the object B is fully submerged in the water. So is the bouyance force equal the force apply by object A on object B? Please help.
  34. R

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    The buoyancy force is defined as the difference in pressures on the top and bottom of an object submerged in a fluid. When an object is pushing on the top surface of a fluid from underneath, the force of surface tension begins to act downward on the object, preventing it from breaking the...
  35. P

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    Homework Statement I've attached the problem I'm kind of lost as to how to even start this problem. For part ) I think my first approach is to sum up forces in the vertical direction. Which would yield F-W1-W2=ma=0 Where: F=buoyancy force W1=weight of submerged volume of oak...
  36. K

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    Archimedes's principle says that the buoyancy force would be equal to the weight of the fluid displaced by the body. It's easy to work with the completely submerged object since I just need the whole volume of the object to as it is equal to the volume of displaced fluid. However, if an object...
  37. S

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  38. D

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    Homework Statement A boat full of people lazily drifts along in Lake Superior. The boat has a mass, including people of 15,450kg. A. What is the buoyant force? B. What is the volume of the displaced liquid in cubic meters? The Attempt at a Solution A. Fb=mg Fb=? m=15,450kg g=...
  39. C

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    How can they be equal at that point? And where is the force that the string applies. By the way Fk = Buoyancy Force
  40. M

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    Hi, I'd like to calculate the buoyancy force exerted on a gate in an intake channel (see picture in attachment). Since the gate is not totally surrounded by water, I guess that the normal buoyancy formula doesn't apply here, am I right ? Is there a rule of thumb for this kind of problem ...
  41. F

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  42. J

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  43. F

    Buoyancy Force question, having hard time starting.

    Homework Statement The 3m plank shown in section has a density of 800 kg/m3 and is hinged about a horizontal axis though its upper edge O. Calculate the angle theta assumed by the plank with the horizontal for the level of fresh water shown. Homework Equations Whats wrong with the editor...
  44. A

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  45. L

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  46. A

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  47. T

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  48. K

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  49. E

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  50. H

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