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. MatinSAR

    Buoyant force acting on an inverted glass in water

    My answer : According to the question, the glass and the air inside it entered the water. Let's assume that the net force becomes zero at a moment, that is, the sum of the weight force and F is equal to the buoyancy force. By going down in the water, the gas volume decreases, so the buoyancy...
  2. A

    I What happens to the buoyant force at the surface of the water?

    What happens to the buoyant force at the surface of the water for an object? The buoyant force should be greater than the weight of the object if the object were to float up but once the object floats to the surface, there is no more acceleration upwards which means the buoyant force = weight of...
  3. 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.
  4. E

    Buoyant Force and Archimedes' Principle

    Hello there, I have a quite different approach on answering the part c and d of this problem. Can you guys help me to see if my method is acceptable or logical? (I got the same answers as the back of the textbook, however my methods differ from the solutions manual). Here is my answer/attempt at...
  5. Hamiltonian

    Doubt related to buoyant force and fluid thrust force

    (this problem is slightly modified from the original problem.) There is a contact force(friction and normal force) between the wedge and the walls of the container and there is a fluid thrust acting on the side of the wedge in contact with the fluid( this force is normal to the slanted surface...
  6. K

    Buoyant force and a water column

    Recently I viewed the Searaser video at After watching it, I feel the numbers given in the video could be wrong but not sure. So I'd like to know how I can calculate the amount of water pushed by the Searaser device at some height in one wave shot e.g. at 100 ft, 200 ft and 300 ft above sea...
  7. tjholi

    Derivation of the Emptying Time for a buoyant box to drain

    Problem Statement: I am having trouble deriving the expression from the initial equations. (Calculate the emptying time considering Volume conservation) Relevant Equations: Q=A*sqrt(b(H-h(t)) And we have dh/dt =Q/S (conservation equation) and we have to obtain h/H = 1-(1-t/te)^2 with te=...
  8. Logic hunter

    B Direction of buoyant force on sunken object?

    We say that buyont force act upwards (in usual cases) and that the normal force exerted by the base of a container (of liquid) on a object is less than its true weight, so a weghing machine will give smaller reading (in terms of value) than expected. But suppose a cube sinks in water. Now water...
  9. Y

    I Thought experiment on buoyant forces on a system

    I'm a former physics student and I've been thinking about an interesting problem that eventually led me to the following thought experiment that I'm having trouble resolving. Imagine a two-compartment system, where one compartment is filled with He gas, and the other is filled with standard...
  10. C

    Buoyancy of Balls A and B in a Swimming Pool

    Homework Statement Balls A and B of equal mass are floating in a swimming pool, as shown below. Which will produce a greater buoyant force? a. Ball A b. Ball B c. Force will be equal d. Impossible to determine without knowing the volume of each ball In the picture given, it is clear that...
  11. V

    Buoyant force on a submerged body

    Homework Statement A hemispherical portion of radius R is removed from the bottom of a cylinder of radius R. The volume of the remaining cylinder is V and its mass is M. It is suspended by a string in a liquid of density ρ where it stays vertical. The upper surface of the cylinder is at a depth...
  12. C

    MHB Calculate Buoyant Force of 2.00L Helium Balloon

    Calculate the buoyant force (in N) on a 2.00 L helium balloon. Work: Upward is Buoyant Downward is weight due to gravity $$\Sigma F=0$$ $B-mg=0$ $B=mg$ $\rho=m/V$ $m=\rho*V$ $B=\rho_{fluid}*V_{fluid}*g$ I am stuck on $m=\rho*V$. Am I missing something?
  13. G

    I Calculating the rate of rise of air

    I know there are many factors that affect this question, such as altitude etc, but in its simplest form... - How fast does a parcel of air rise if, say, the parcel of air is at 5 degrees centigrade in a room filled with air at 0 degrees C. (Maybe I should be using Kelvin?). Just need to get an...
  14. K

    Calculating the electrical energy of a buoyant force

    Should be a simple question. Hopefully... Buoyant object rising in water pulls on a cord attached to a generator. How much power is generated? Object Volume = 100m2 Liquid is plain water = H2O Depth = 100m I understand that Archimedes Principle is the buoyant force = the weight of the...
  15. Superman123

    Is the buoyant force proportional to the mass?

    1. The task is to figure out if the buoyant force is proportional to the mass of the object that is being sunk in the fluid (the fluid in this case water). I had an answer and that the buoyant force is proportional to the mass, but my answer only took into consideration objects with the same...
  16. T

    The Buoyant Force on a Man: What is the Ratio to His Weight?

    Homework Statement A certain man has a mass of 85.0 kg and a density of 995 kg/m3 (exclusive of the air in his lungs). (a) Calculate his volume. .0854 m3 (b) Find the buoyant force air exerts on him. N (c) What is the ratio of the buoyant force to his weight? (buoyant force / weight)...
  17. 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...
  18. Ian Baughman

    I Buoyant Force and what densities to consider?

    So I know FB = ρDF × g ×VDF when we submerge an object in a fluid such as water but what if we were talking about a hot air balloon? In this case would it be correct to use FB = (ρin - ρout) × g ×Vobject? Where ρin = density inside balloon and ρout = density outside balloon. If this is...
  19. A

    I How come there is a buoyant force?

    Hello. Suppose you have an incompressible fluid in a column of infinite height and area and that is in a vacuum. The formula for the net pressure at any point is hpg, where p is the density and h the height. The formula comes from considering that at h = 0, the net pressure is zero, but that at...
  20. A

    I How does gauge pressure affect the buoyant force on a submerged object?

    How can we relate gauge pressure (pg) to the buoyant force , in a completely submerged object? is it as follow: Buoyant force= pg.Area +Density(water).volume.gravity ?
  21. M

    B Buoyant Force: Pressure, Gravity & Upward Thrust

    how does pressure on a submerged body in water experience an upward thrust ,when the cause of pressure is gravity, which can only be downwards
  22. C

    Buoyant Force in High vs Low Density Liquids: Homework Statement & Solution

    Homework Statement When the object is submerged into higher density liquid compared to the lower density liquid, the object will floats higher in the liquid... Homework EquationsThe Attempt at a Solution The object will be more stable, right? Can someone explain why it...
  23. lucynemes86

    If the object floats, the buoyant force will be equal to its

    Homework Statement If an object floats, the buoyant force will be equal to its: a. Mass c. Specific Gravity b. Density d. Weight Homework EquationsThe Attempt at a Solution Is it weight?
  24. G

    Buoyant force and ambient air pressure

    I wanted to inquire as to a thought I have on a situation with buoyancy and air pressure. In the first slide (slide 1), I am showing a situation where a buoyant object is pushing up on a support wall through a fixed wall that has water in it. Here are the factors: 1. The net buoyant force of...
  25. J

    Exploring Buoyancy: How Ball Moves in Water, Oil and Moon Gravity

    Homework Statement 1a)How does a ball floating 50% in water move when a LARGE amount of oil is added? 1b) if something denser than water were added, how would the ball move? 1c) if the exp. was done on the moon with different gravity and no atmosphere, how would the ball move?Homework...
  26. Titan97

    Buoyant Force Question: Cylinder and Cone in Water - Homework Solution

    Homework Statement (I made this question myself) What happens to the cylinder (in orange color) kept inside a beaker filled with water like this: The cylinder just passes through a hole on the bottom of the beaker. Assume that water can't leak through the hole. Will the cylinder move up? Or...
  27. Ethan Godden

    What Happens to the Buoyant Force on a Beach Ball When Submerged?

    Homework Statement A beach ball is made of thin plastic. It has been inflated with air but the plastic is not stretched. BY swimming with fins on, you manage to take the ball from the surface of a pool to the bottom. Once the ball is completley submerged, what happens to the buoyant force...
  28. C

    Buoyant Force acting on a sphere

    Homework Statement A sphere of radius 10.0 cm floats in equilibrium partially submerged in water with its lowest point 5.00 cm below the water's surface. (a) What is the buoyant force acting on the sphere? Homework Equations F = pvg The Attempt at a Solution F= 1000 * v * 9.8 V=...
  29. C

    What is the volume of the wood immersed in the water-filled flask?

    A piece of wood with a density of 730 kg/m3 is tied with a string to the bottom of a water-filled flask. The wood is completely immersed, and the tension in the string is 1.09 N. Find the volume of the wood.
  30. B

    Archimede's Principle and Work due to buoyant force

    Homework Statement A flotation device is in the shape of a right cylinder, with a height of 0.323 m and a face area of 4.81 m2 on top and bottom, and its density is 0.460 times that of fresh water. It is initially held fully submerged in fresh water, with its top face at the water surface. Then...
  31. J

    How much does the McLane sampler weigh in water?

    I am trying to design the housing for an electrical core so that it remains buoyant on the surface but can be neutrally buoyant at desired depths through ballast tanks. I am trying to figure out what the core and housing may typically weigh through specifications online of ones already made...
  32. J

    Absolute pressure, buoyant force, tension problem

    Homework Statement A diving sphere has a mass of 150,000.0kg and an external diameter of 7.000 meters. The sphere is anchored with a cable on the bottom of an ocean, at a depth of .8000 km. The density of sea water is 1025.0kg/m3. A) What is the absolute pressure at that depth in pascals? B)...
  33. personage

    Buoyant Force of a block in water

    A flask of water rests on a scale that reads100 N. Then, a small block of unknown material is held completely submerged in the water. The block does not touch any part of the flask, and the person holding the block will not tell you whether the block is being pulled up (keeping it from falling...
  34. BrainMan

    Is the Bar of Gold Real? Using Buoyancy to Determine Density

    Homework Statement A man decides to make some measurements on a bar of gold before buying it at a cut-rate price. He find that the bar weighs 2000 N in air and 1600 N when submerged in water. Is the bar gold? Homework Equations B = density* V* g The Attempt at a Solution I tried to use the...
  35. BrainMan

    Calculate the buoyant force on a solid object made of copper

    Homework Statement Calculate the buoyant force on a solid object made of copper and having a volume of 0.2 m^3 if it is submerged in water. Homework Equations B = pVg The Attempt at a Solution I followed the above equation and did B = (8.92 x 10^3 kg/m^3)(0.2 m^3)(9.8 m/s^2) B = 1.74 x 10^4...
  36. AdityaDev

    Understanding Buoyant Force: Exploring FBD and Equations in Accelerating Vessels

    If I have an object of mass m tied to the lower surface of a vessel having a liquid and the vessel accelerates upwards... From FBD of object, Buoyant force acts upwards, mg down, pseudo force downwards (frame of reference is vessel) T down. Here's the doubt. Why is F(buoyant force) = Vp(g+a)...
  37. C

    Conceptual question regarding buoyant force

    A penguin floats first in a fluid of density p0, then in a fluid of density 0.95p0, and then in a fluid of density 1.1p0. a) Rank the densities according to the magnitude of the buoyant force on the penguin, greatest first. b) Rank the densities according to the amount of fluid displaced by the...
  38. E

    Buoyant Force and Archimede's principle

    Homework Statement A balloon used for a physics experiment has a radius of 3.15m and is filled with helium. The total mass of the balloon is 15.2kg and the density of the surrounding air is 1.28 kg/m3. a. How much buoyant force is acting on the balloon? b. When the balloon is released from the...
  39. H

    Is My Buoyant Force Calculation Correct?

    Homework Statement A recreational (open) hot air balloon (i.e., Pinside is approximately Poutside) has a volume of 2107 m3 when fully inflated. The total weight of the balloon, basket, ballast and pilot is 1832.6 N (412 lbs). By how much must the density of the air in the balloon be smaller...
  40. T

    Buoyant Force & Gravity Force find Fnet (Newtons Second Law)

    Homework Statement A 4 kg book sinks a distance of 5m in the ocean starting from rest. The volume of the book is 0.00266666666666667 m^3. We assume the velocity of the Earth is zero during the whole process, and gravity and the bouyance force are the only interactions of the book. (Note...
  41. S

    Finding a buoyant force on an object

    Homework Statement You are given a density of a fluid, volume of the rubber stopper and density of the rubber stopper. If a rubber stopper sinks to the bottom of the beaker filled with this fluid and stays there, find the buoyant exerted on the object by the fluid. Homework...
  42. Greg Bernhardt

    Buoyant Force Definition & Explanation - Archimedes' Principle

    Definition/Summary Buoyant force (or buoyancy) is the net force on a body caused by the pressure differences in the surrounding medium caused by gravity. Buoyant force acts through the centre of gravity of the displaced fluid (the centre of buoyancy). If the centre of buoyancy is not on...
  43. H

    Buoyant force on a submerged cube

    So a cube submerged in water will experience pressure on all six sides, and the pressure on the bottom will be greater than the pressure at the top (assuming there's gravity) and the cube will float to the top given that it has a low enough density. Something I'm curious about is what would...
  44. K

    The Effect of Temperature on Buoyant Forces

    Hi, I came across this question in a physics review pamphlet: Homework Statement A balloon for a county fair is designed to carry four 100-kg passengers when it is expanded to its maximum volume. The designers assumed the balloon would operate in ordinary spring temperatures. If, on...
  45. Nathanael

    Buoyant Force at the bottom of a pool?

    Apparently, Buoyant Force is caused by a difference in water pressure. So am I correct in thinking that this means if you have a cube (chosen so that buoyant force does not come from the sides, just from the bottom) at the bottom of a pool, there would be no buoyant force? How could there...
  46. A

    Calculating Work Done by Upward Buoyant Force on Hot Air Balloon

    Homework Statement A hot air balloon of mass 425 kg ascends from the ground level and accelerates at a rate of 1.25 m/s/s for 25 seconds. How much work is done by the upward buoyant force on the balloon? (conceptual) A certain amount of work W is required to accelerate a car from rest to a...
  47. N

    What does the buoyant force depend on?

    Homework Statement What does buoyant force depend on: the mass of the object, its volume, its density, or the material from which it's made?2. Relevant concepts Archimedes' principle: Buoyant force = weight of displaced liquidThe Attempt at a Solution I just don't quite understand this concept...
  48. A

    Calculating Buoyant Force for a Suspended Cylinder in an Unknown Fluid

    Homework Statement An aluminum cylinder is suspended in an unknown fluid by a spring scale. Outside the fluid, the object weighs 20N. In the fluid, the scale reads 10N. What is the buoyant force on the cylinder? Homework Equations Fb= pVg, p= density of fluid, V= displaced volume, g=...
  49. A

    What Weight Can a Wooden Block Support Before Sinking?

    Homework Statement Wooden block: Mass: .095. Weight: .931. Volume: 2.03E-4. Density: 467.98. Buoyant force of block: 1.9894. - Predict how much weight the wooden block can support before sinking. Homework Equations W= m1g + m2g The Attempt at a Solution Not sure where to...
  50. A

    AP physics- finding buoyant force

    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. Find buoyant force on the block. Homework Equations Fb= p x V x g. p= density of fluid, density of water...
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