Buoyant Definition and 167 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. A

    Calculating Buoyant Force for Submerged Materials

    Homework Statement A cubic meter of some material has a mass of 8600 kg. The block of material is lowered into a lake by a strong cable until the block is completely submerged. What is the buoyant force on the block? Homework Equations Fb=p x V x g. P= density of fluid, density of...
  2. A

    How does oil affect the buoyant force of a block of wood versus water?

    Hi, I'm having trouble with a conceptual problem: Homework Statement There is a block of wood in a jar with water; it is floating. If I pour oil into the container, obviously the oil sits at a layer above the water. The wooden block touches the water, oil and air now. Did the volume of the...
  3. S

    Buoyant force and volume submerged

    Hello, This is bugging me so I'd appreciate any help. I have two objects made from the same material with the same mass. The base of Object 2 is 3 times that of Object 1. If I place the objects base first in a liquid, why is the buoyant force on them equal? What here is what I'm...
  4. D

    Buoyant Force of a ship and water

    Homework Statement Consider a ship that is floating in fresh water. The bottom of the ship is a depth of (df) below the surface. If the same ship is floating in sea water, the bottom of the ship (ds) below the surface. Given that the density of sea water is greater than the density of fresh...
  5. T

    Need help finding the buoyant force in this problem

    Hi everyone, This question is killing me, I don't know why but I just can't get my head around it. Homework Statement A team of 6 scouts plans to cross a lake on a raft they designed. The scouts have wooden beams with an average density of .80. The beams measure 30cm x 30cm x 3m. The...
  6. R

    Calculating the buoyant force on a cube

    Homework Statement A cube, with a volume of 0.78 m3 is submerged in a swimming pool. What is the buoyant force acting on the object if it has a mass of 328 kg? Homework Equations The Attempt at a Solution
  7. orlan2r

    The upward buoyant is a conservative force?

    If the upward buoyant is a conservative force, then it is possible to assign a numerical value for the potential at any point. See this please (altought this is in spanish): http://cpreuni.blogspot.com/2012/03/energia-potencial-hidrostatica.html...
  8. K

    Action and reaction pair for buoyant force.

    According to Newton 3rd law, every action will have a reaction. but how about buoyant force? if the buoyant force is the force of water on object then the reaction force will be the force of object on water. but is it other name for this force? thanks for reply.
  9. P

    Static Equilibrium with buoyant force. High difficulty.

    Homework Statement I attached the original problem and a diagram I made with the variables on it. Basically we have a rod hanging from a wire at an angle theta with tension and the other end floating in water. So there is T=tension, weight=w=mg and buoyant force=F_b. I introduced symbols for...
  10. U

    Buoyant force in seawater and freshwater

    Homework Statement I'm doing a problem that is likely a common one. It is comparing the buoyant force of an object in seawater to that of the same object in freshwater. which has more buoyant force acting on it? Homework Equations not sure of equations at this stage, and we are just...
  11. G

    How Do You Calculate Buoyant Energy?

    Buoyancy is an artifact of gravity that moves mass up away from the center of the Earth in a fluid whenever the specific gravity of the object is less than the surrounding fluid. Buoyant Force is calculable. Gravitational Potential Energy is calculated by the formula: mgh Is Buoyant Potential...
  12. K

    A 1.2 kg block is pushed under water and held there. What is the buoyant force?

    A 1.2 kg block is pushed beneath the water and held there. What is the buoyant force (in N) acting on the block? (a) not enough information to answer (b) 11.76 (c) 1.2 (d) 14.56 (e) none of these Since buoyant force is equal to the wt of the water displaced, I used the equation w=mg...
  13. A

    Solving for Density: Buoyant Force & Force of Gravity

    Homework Statement Homework Equations density = mass/volume FB = rhoo vog for a floating object F = mg The Attempt at a Solution Buoyant force = force of gravity due to the board rhoo vo = m vo = area*height submerged vo = 2.4*.05 vo = 0.12 m^3 volume of the board =...
  14. L

    Buoyancy/Gravity Concept - How to insert buoyant weight into water column?

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  15. L

    The buoyant force of air? Concept question

    if we weigh a metal object which is immersed in water, we have to correct the buoyant force for water. Air is a fluid. Why don't we correct for the buoyant force of the air when we weigh it in the air? What i think it may be is because we have a set atmospheric pressure being 1.013 Kpa in...
  16. B

    Help finding potential energy of submerged buoyant object

    object is the shape of a ball 300 feet in diameter. (~100m) object weighs 100,000 lbs (~50,000 kg) object is at a depth of 1000 feet (~300m) I just need a rough estimate - you can use meters or feet or whatever, i don't care. salt or fresh water is fine but if there is a choice salt...
  17. J

    What is the density of the liquid?

    Homework Statement A block of copper having a density of 8.90 g/cm^3 has an apparent mass 120g in water and 116g when submerged in a liquid. What is the density of the liquid? Homework Equations The Attempt at a Solution Don't know if there's a better method, but here is the way I did it...
  18. M

    Buoyant force and water displacement

    couple question regarding buyoancy and water displacment 1. so if you have a pond w/ ρ=1.2 and part of the water in that pond turns into ice will the level decrease increase stay the same? (i think it will stay the same) 2. same pond w/ a ρ=1.2 and you add ice cubes w/ a ρ= 1.2 the cubes...
  19. H

    Are air resistance and buoyant force same

    And does air resistance depend upon surface area?
  20. G

    Making sure my intuition is correct about buoyant force

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  21. D

    Buoyant force of an aluminum block

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  22. J

    Buoyant force of air negligible?

    Buoyant force of air negligible?? Why are we justified in assuming that the buoyant force on an object in air is generally negligible. Can you please explain using some example equations? Many thanks.
  23. J

    Does the buoyant force change in different fluids?

    Given two objects of identical density and weight, one is floating on water and the other is floating on saltwater. Is the magnitude of buoyant force acting on the object the same in the two situations? From my thinking, it seems that the weight of the object in the the two situations should not...
  24. G

    What is the Velocity of the Larger Cube as the Smaller Cube Melts?

    Homework Statement Two cubes made of the same material and edge length 'a' and '2a'. They are placed in a liquid of density σ such that the larger cube is completely submerged when the smaller cube is placed on top of it and the lower surface of smaller cube is just touching the surface of...
  25. A

    Buoyant Force Problem: Fraction of Length in More-Dense Liquid | Solution

    Homework Statement A less-dense liquid of density ρ1 floats on top of a more-dense liquid of density ρ2 . A uniform cylinder of length l and densityρ, with ρ1< ρ<ρ2, floats at the interface with its long axis vertical. What fraction of the length is in the more-dense liquid? Homework...
  26. S

    Understanding the buoyant force in archimedes principle

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  27. K

    Buoyant Force of sand and steel

    I am just not too sure how to start this equation. The answer was given (i.e. Vc = 3698 cm^3), but I just need to know the steps on how to get to the answer. A sand core used to form the internal surfaces of a steel casting experiences a buoyancy force of 225.63 N. What is the volume of the...
  28. K

    What is the net force acting on a submerged juniper wood plank?

    1. A juniper wood plank measuring 7 ft by 0.5 ft by 0.1 ft is totally immersed in water. What is the net force acting on the plank? () 2. a) F top = P top * A top(l x w) ; b) P bottom = P top + (Dw * h); c) F bottom = (F top) + (Dw*l*w*h); d) Fb = F bottom - F top e) Weight = Dw * l * w...
  29. C

    Why Does Density Matter for Buoyant Force?

    Why is the fluid's density taken in the buoyant force? If the system is in equilibrium, then isn't the buoyant force compensating for the weight of the object? In such a case, shouldn't the density of the object be considered? Please help!
  30. D

    Solve Buoyant Force/Simple Harmonic Motion Homework

    Homework Statement A light balloon filled with helium of density 0.18 kg/m^{3} is tied to a light string of length 1.51 m. The string is tied to the ground, forming an “inverted” simple pendulum as in the figure. The acceleration of gravity is 9.8 m/s^{2}. If the balloon is displaced slightly...
  31. C

    Buoyant Pressure problem number 7

    Homework Statement When a beaker partially filled with water is placed on an accurate scale the scale reads 22.0g. If a piece of wood with a density of 0.905 kg/dm^3 and volume of 2.10 cm^3 is floating on the water, what will the scale read Homework Equations m=Density times volume The...
  32. C

    Pressure, Buoyant Force problem 8

    Homework Statement When a beaker partially filled with water is placed on an accurate scale the scale reads 22.0 g. If a piece of metal with a density of 3.800 kg/dm^3 and volume of 2.40 cm^3 is suspended by a thin string so that the metal is submerged in the water does not rest at the bottom...
  33. C

    Pressure, Buoyant Force problem

    Homework Statement A cube of metal(density=6.00 kg/dm^3) has a cavity inside it. It weighs 2.40 times as much in air as it does when completely submerged in water. What fraction of the cube's volume is the cavity? Homework Equations Density=m/v Fb=Density of fluid times volume of fluid...
  34. C

    Pressure, Buoyant Force problem

    Homework Statement A solid cube of unkown material is floating upright in water so that 1/4 of the cube is above the surface of the water. What is the density of the material? Homework Equations Density=m/v F=mg Fb=density of fluid times volume of the fluid times g P=density times...
  35. C

    Pressure and Buoyant Force problem

    Homework Statement A downward force of 18.0 N must be applied to a woman weighing 480.0 N to keep her completely submerged in water. What is the density of her body Homework Equations Density=m/v Fb=density of fluid times volume of fluid times g Fb=F(bottom) minus F(top) The Attempt...
  36. C

    How Much Force to Keep Foam Underwater?

    Homework Statement A block of foam plastic has a volume of 25.0 cm^3 and a density of 0.800 g/cm^3. How large a force is required to hold it under water? Homework Equations F=mg, m=density times volume The Attempt at a Solution I first converted 25.0 cm^3 into 2.5 times 10^-5m^3 and...
  37. C

    How Do You Calculate Apparent Weight and Buoyant Force?

    Homework Statement 1) A 0.0200 kg metal(density=4.000 g/cm^3) object is suspended from a thead and is submerged in a jar of oil(density=1.500 kg/dm^3) What is the apparent weight in the thread) For this one I keep doing this one but I think I am getting it wrong because of my conversion...
  38. D

    Buoyant force same for all object of same volume?

    Rank the buoyant forces exerted on the following five objects of equal volume from the largest to the smallest. Assume the objects have been dropped into a swimming pool and allowed to come to mechanical equilibrium. If any buoyant forces are equal, state that in your ranking. (Use only ">" or...
  39. J

    Simple Harmonic motion of buoyant force

    Homework Statement a cylinder is in a deep pool of liquid. it has mass 2kg, diameter 10cm, height 15cm. the density of the liquid is 1.5g/cm^3. The cylinder is submerged up to 12cm, and let go. Show that the resulting motion is simple harmonic. Homework Equations x=Acos(wt)...
  40. 2

    Calculating Buoyant Force of Balloon System

    Homework Statement A hot air balloon experiences an acceleration of 110 m/s2 [down]. The total mass of the balloon is 315kg What is the buoyant force of the system? aballoon = 110m/s2 agravity = 9.8m/s2 mtotal = 315Homework Equations Fgravity = 315 * 9.8 = 3087 Fnet = 315 * 110 = 34650The...
  41. C

    An flying object needs lift,but this lift is like buoyant force in air?

    An flying object needs lift,but this lift is like buoyant force in air?? just like when any object is immersed in water it feels lighter because a net upward force is acting on it same way do net upward force is acted by air when any object is flying in air?? and that net upward force is more...
  42. R

    Archimedes Principle, Buoyant Force

    Homework Statement Your local king has asked you to determine the density of a crown. If you suspend it from a spring scale, the scale reads 8.33 N. If you then lower the statue into a tub of water, so that it is completely submerged, the scale reads 7.81 N. What is the density of the...
  43. B

    What best describes the buoyant force on the objects?

    Homework Statement Two objects made from the same material with the same mass are placed in a liquid, base first. The base of Object 2 is three times that of Object 1. What best describes the buoyant force on the objects? A) Object 1 has a greater buoyant force acting on it because it...
  44. A

    What is the tension in the rope holding a helium-filled balloon at a party?

    Homework Statement Bozo the clown brings an empty birthday balloon with a mass of 0.0120 kg to your party. Bozo fills the balloon with helium of density 0.181 kg/m3. The filled balloon is a perfect sphere with a radius of 0.500 m. Bozo ties the balloon to a rope and holds is so that it is...
  45. M

    Calculating Buoyant Force of Ice in Water: Homework Practice

    1. a)Homework Statement What is the buyant force on 0.90kg of a block of ice floating on liquid water? B) What is the buyant force on 0.90kg of ice held completely submerged under water? Homework Equations Fbuy+mg=0 Fb=mg The Attempt at a SolutionFb=.90kg * 9.18m/s^2=8.8 so i...
  46. W

    Do Buoyant Forces Depend on the Material's Density?

    Homework Statement A 20cm^3 block of lead and a 20cm^3 block of copper are completely under water. Each is suspended by a thread so that they both hand at the same height in an aquarium filled with water. Which of the following is true? a) The buoyant force is greater on the lead than on...
  47. B

    What is the magnitude of upthrust force on a metal ball immersed in water?

    a metal ball weighs 1 N in air and 0.8 N when totally immersed in water. find magnitue of upthrust force.
  48. K

    Buoyant Force and Massive G force protection?

    I am interested in determining if a method can be devised to create a safe zone for humans to be accelerated at very high G force. I watched a very interesting experiment where a water filled balloon was submerged in container of water and had 60 g's of force applied via a centrifuge. The...
  49. C

    Buoyant force on a balloon is equal to the mass of air it displaces

    Homework Statement The buoyant force on a balloon is equal to the mass of air it displaces. The gravitational force on the balloon is equal to the sum of the masses of the balloon, the gas it contains, and the balloonist. If the balloon and balloonist together weight 175 kg, what would the...
  50. D

    How Do Buoyant Forces Affect Submerged Objects?

    Homework Statement note: It may make things easier to assume that g=10 m/s^2 Suppose that I have a 100 gram chunk of metal with a density of 4 grams per cubic centimeter. 1)What is the mass of water displaced by this chunk of metal?(give your answer in kg) 2)what is the buoyant force on the...
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