Buoyancy Definition and 413 Threads

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. B

    Calculating Volume Using Buoyancy

    Homework Statement http://img524.imageshack.us/img524/1743/buobu1.png Homework Equations Vsphere= pi/6(d^3) W = mg The Attempt at a Solution Wc + Wo' = 0.650 N Wc' + Wo' = 0.550 N I'm unsure of how to do this because it asks to find the volume ONLY BY USING THE two weight...
  2. Saladsamurai

    Stupid Buoyancy, this is getting ridiculous.

    [SOLVED] Stupid Buoyancy, this is getting ridiculous. A hollow sphere of inner readius 8cm and outer radius 9cm floats half submerged in a liquid of density 800kg/m^3. What is the mass of the sphere and the density of the material it is made from. \sum F=0 \Rightarrow mg-F_b=0 \Rightarrow...
  3. 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:
  4. M

    How Can We Improvise Buoyancy Control for a Mini-Submarine Without Using a BCD?

    Hello! my group are designing ofwhat we called a mini-submarine..it is somewat like an underwater surveoyr..but in a submarine form of construction. our adviser told us, that we could use a buoyancy controlling device to address our problem of getting our mini-submarine to float and...
  5. M

    Buoyancy Systems in Closed & Semi-Closed Systems

    A few months ago I was seeking an answer to a question about using buoyancy in a closed or semi closed system to generate electricity on anything of a useful scale. I came across the thread below on these forums. https://www.physicsforums.com/showthread.php?t=148606" I was exploring this...
  6. K

    Energy Violation Puzzle - The use of Negative Buoyancy to run a turbine

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  7. M

    Good old question about buoyancy and riser in oil industry

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  8. D

    Classic buoyancy demo discussion on fluid dynamics

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  9. F

    What is the buoyant force of water on the block?

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  10. S

    Solving Buoyancy Problem: Find Depth of Cylindrical Rod

    I'm struggling a bit on a homework problem where a cylindrical rod of radius r is floating in equilibrium in water. The bottom 4 cm of the rod has a density of 2000 kg/m^3 while the other 6 cm of the rod has a density of 900 kg/m^3. The question asks what the depth of the bottom of the rod is...
  11. M

    Buoyancy Formula (Fb, mdg, T etc)

    Homework Statement Hi Everyone! I'm new to this forum and a beginner at physics so I hope to get some help! :-) I am doing a report for a lab we did a few weeks ago, and it happens to be one where I was feeling awful and obviously not paying enough attention! We were using a rubber...
  12. T

    Particle System (Gravity, Buoyancy and Drag),

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  13. H

    What is the direction of the buoyancy force?

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  14. K

    What Are the Key Principles of Hydrostatics and Buoyancy?

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  15. C

    Does Shape Affect the Tension in Strings Suspended in Water?

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  16. C

    How Much Energy to Create a Vacuum in 1 Cubic Meter at Sea Level?

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  17. C

    Is it plausible to generate electricity through buoyancy?

    Is it plausible to use the buoyancy of air in a controlled column of water to turn a drive shaft and, by extension, generate electricity through a low RPM/high torque turbine? See attached PDF for possible configuration. Only energy input would be to compress air tanks located below the water...
  18. T

    Solve Buoyancy and Tension Homework Problem

    I'm not very good at fluids.. and I was wondering if someone could check my work if they had time ^^;; What I did makes a lot of sense to me though.. so usually that's a sure sign that I did it wrong. Thanks for your time ^^; Homework Statement A 10 kg piece of lead is completely...
  19. J

    Calculating Buoyancy of Steel Boat

    [b]1. The bottom of a steel "boat" is a 4.00 m x 10.0 m x 4.00 cm piece of steel (rho=7900). The sides are made of 0.490 cm-thick steel. Homework Equations Fbouy = Wobj The Attempt at a Solution i have gone around in so many directions that I am pretty much lost...is okay to...
  20. D

    Buoyancy Problem Help: Understanding Changes in Water Level and Scale Readings

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  21. M

    Buoyancy of wood block problem help

    A block of wood floats on water. A layer of oil is now poured on top of the water to a depth that more than covers the block. If 91% of the wood is submerged in water before the oil is added, find the fraction submerged when oil with a density of 883 kg/m3 covers the block. (Do not neglect the...
  22. B

    How Do You Calculate the Radius of a Submerged Iron Ball?

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  23. B

    Finding the density of a liquid with buoyancy

    http://spock.physast.uga.edu/prtspool/bearhug_uga_printout_1163471689_22380_10.pdf A rectangular block is gradually pushed faced-down into a liquid. The block has height d; on the bottom and top the face area is A=5.67 cm^2 . Also shown is a graph that shows the apparent weight W_app of the...
  24. K

    Is There Buoyant Force at the Base of an Object in Water?

    I was doing a test and the there is a picture with the box at the bottom of the water.First they ask me to find the weight.I found it.Then later they ask me to calculate the buoyant force.My teacher said that there is no buoyant force at the bottom.But the Archimedes Principle states that the...
  25. Jorrie

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  26. W

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  27. X

    Buoyancy and Density and Buoyancy

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  28. D

    Adding buoyancy to 3D integration of diffusion equation

    I have managed to create a simple CFD model to integrate in 3D the heat/diffusion equation, namely dT/dt = k d2T/dx2. This works fine, but I would now like to add the effect of 'less dense rising' assuming the gas is operating in air -- I think I'm right in saying that's called buoyancy (I'm...
  29. P

    Calculate Percent Volume Submerged: Buoyancy Problem

    A rectangular piece of wood ( 5.0 X 4.0 X 3.0 cm) and mass 42g is thrown into the oceon, how do i calculate the percentage of volume floating above the water?
  30. C

    Buoyancy and Archimedes' Principle (helium ballon carrying cargo), HELP

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  31. P

    What Is Buoyancy? Explaining Its Uses

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  32. T

    Buoyancy - how much does this person drink?

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  33. J

    AZING!Calculating Vacuum Buoyancy: Maximum Weight a Vacuum Cleaner Can Lift

    A very powerful vacuum cleaner has a hose 2.75 cm in diameter. With no nozzle on the hose, what is the weight of the heaviest brick that the cleaner can lift? So I need to find the pressure and then multiply by the area of the hose... but if P= density*g*h... would I use the density for air...
  34. P

    Finding the Density of a Wooden Cube Submerged in Water

    "A solid wooden cube, 30.0cm on each edge, can be totally submerged in water if it is pushed downward with a force of 54.0N. What is the density of the wood?" I know so far that the buoyant force would equal the 54N, but I don't know how to get the mass of the cube to determine the density...
  35. T

    Calculating Buoyancy to Find a Polar Bear's Mass

    Buoyancy ... HELP NEEDED A polar bear is partially in water and partially on a slab of ice. The ice sinks to some extent. Calculate the bear's mass. How can this be done? I know it comes down to forces but how . . .
  36. D

    Buoyant Force and Floating Objects in Accelerated Water

    Hi I have this problem and I don't fully understand how to get started, so please help. Thanks A bucket of water is accelerated upward at 2.4g. What is the buoyant force on a 3.0-kg granite rock (SG=2.7) submerged in the water? Will the rock float? Why or why not? I know that it would...
  37. S

    Solve Force & Buoyancy: 1000N to Lift Concrete Block Out of Water

    Can anyone help me out. I don't know of any equation that i can us eto solve this A force 1000N is required to raise a concrete block to the surface of a fresh wate rlake. what is the force required to lift it out of the lake. I know that i need to figure out what the weight outside of wate ris...
  38. Y

    How to Derive the ODE for a Floating Spherical Float?

    I am trying to set up a first order differential equation to express the oscillation of a spherical float floating on water. I know that the two forces acting on the spherical float is force buoyancy and gravity and there is two density (density of water and density of the sphere). So first what...
  39. A

    Does Salt Water Affect the Weight of Displaced Water?

    hi all, I'm completely stuck on this question I've got no idea how to tackle it, I've looked back at previous examples and i don't know how this fits in with what I've see before. heres the question a boat floating in fresh water displaces water weighing 35.6kn what is the weight of the...
  40. R

    Black Hole: Infinite Density, Infinite Buoyancy?

    If a black hole has an infinite density, then one would think that anything would float inside of it. And since it's infinitely dense, the object(s) being pulled in would have an infinite buoyancy, causing it to be shot back out of the black hole at a seemingly infinite speed. So why don't...
  41. S

    Buoyancy and Archimedes' Principle

    If an object floats in water, its density can be determined by tying a sinker on it so that both the object and sinker are submerged. Show that the specific gravity, (Density_substance)/(density_water at 4 degrees), is given by (w)/(w_1 - w_2), where w is the object's weight in air alone, w_1 is...
  42. C

    What Causes Buoyancy in Water? Answers and Explanations

    Hello. I have a question or two about buoyancy, in water specifically. What causes buoyancy, if there is something more than the difference of pressures above and beneath an object? What would happen if I put a very thin object in water, so that the difference in pressures would be extremely...
  43. T

    Quick question on fluids and buoyancy

    Hi, i have a quick question regarding the above subject. If F(b) = (rho) (V) (9.8) and F(b) = F(g) and F(g) = (m) (9.8) then wouldn't F(b) = (m) (9.8) ?
  44. H

    Solving Review Questions for Test: Bernoulli's Principle, Buoyancy, and More

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  45. M

    Hot air balloon buoyancy homework

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  46. M

    Buoyancy and Acceleration: Solving for Tension in a Vessel

    1.The tension in a string holding a solid block below the surface of a liquid (of density greater than the solid) is To when the containing vessel is at rest. Show that when the vessel has an upward vertical acceleration of magnitude a, the tension T is equal to To(1+ a/g)?
  47. T

    Buoyancy Equilibrium on a balance

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  48. T

    Calculating Fluid Displacement in a Multi-Fluid System

    i seem to be stuck on a problem which involves fluids of 2 different densities, and an object floating in/on them. A block of wood is floating in a jar of water. oil is then poured onto the water. how can i calculate the mass of "the fluid displaced" when it is composed of 2 different fluids...
  49. C

    Theoretical Question about Buoyancy

    This is a question that's been bothering me for a while. Archimedes's principle states that the weight of the water displaced is equal to the buoyant force. If a frictionless box sinks to the bottom of a lake, does it feel a buoyant force? Buoyancy exists because of a difference in pressure...
  50. J

    Understanding Buoyancy: The Role of Surface Imperfections in Object Submersion

    Place a block at the bottom of a glass cup. The bottom of the block is polished smooth so that no air exists between the cup bottom and the block. Now fill the cup with water. Does the block rise?
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