What is Buoyancy: Definition and 411 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. B

    B Why Do Poached Eggs Stay In The Center?

    I cracked an egg into water and it sank implying the egg matter is average more dense than water. If this is the case, a vortex should force the egg (when on the surface) off to the edges via the difference in centripetal force. Why then does the egg stay in the center when the water is swirling?
  2. Massimo

    Calculate the max speed at which a submarine rises/dives

    I'm trying to figure out how I would calculate the rate at which a submarine (or any vaguely cylindrical object) ascends or descends in seawater given a certain buoyancy or change in buoyancy. For example, if my submarine is 2000 feet below sea level and weighs 393 metric tons (with empty...
  3. B

    Volume's effect on buoyancy: Does pressure increase?

    Hey! Im currently writing a lab on how an increase in the volume of an object will result in an increase of buoyancy force acting on an object. We fully immerse different amounts of clay playing blocks into water (using a string) on top of a scale, and calculate the buoyancy force. The reasoning...
  4. Geospinelsulfid

    I Can buoyancy exchange still happen in a gravitational fed water system with closed hydraulic head?

    Hey Guys, i am currently trying to implement the technology of this paper (https://www.cell.com/joule/fulltext/S2542-4351(23)00360-4)(Extreme salt-resisting multistage solar distillation with thermohaline convection) into a product for my Design Diploma. As the picture hopefully helps to...
  5. A

    B Does the density of a liquid affect the braking of an object in free fall when it lands?

    In mine hypothesis I want to slow down free fall for diffrent density objects in liqudies. I have a stone wich i roughly a denisty of 2,7 g/ml and gold with 19,7 g/ml. They have the same size. Liquied glucose has the density of roughly 1,5 g/ml. In my example, I drop at the same time in a 1...
  6. C

    Pressure at bottom of three differently shaped cylinders

    For this problem, The solution is, However, I thought the answer would be (b) and (e). I choose (b) because from the formula ##P = \frac{F}{A}## then since the vessel A has more water, then the water will have a larger weight ##W_A > W_C > W_B##, therefore from##P = \frac{mg}{A}## we conclude...
  7. 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...
  8. T

    Exploring the Relationship Between Buoyancy and Apparent Mass

    my solution: a) F(upward)=Fb +Fw =(1.3+1.5)X9.8 =27.44N total Mass = 2.8kg b) Volume increased = π(0.2/2)^2 x 1.5/100 =4.7x10-4 m^3 T+Fb =mg T=mg-Fb T=2.2x9.8 -1000 x 4.7x10-4 x 9.8 T=17.4N T is the apparent weight of the rock, so the mass of the rock in the water...
  9. MatinSAR

    Question about Elastic force and Buoyancy force

    Suppose a wooden cube with a side of 10 cm is connected to the bottom of a water container by a spring. If the density of wood is 0.6g/cm^3 and the density of water is 1g/cm^3 , what is the elastic force of the spring? Is it 4N or 6N ? I think the answer is 4N … But the book says it is 6N ...
  10. B

    B What is my apparent weight in water?

    Hi, I was wondering, what is my apparent weight in water? For example, when I tiptoe on land, my calf muscles are pretty much lifting all my body weight. But if I were to tiptoe with just my head out of the water, how much weight would my calves be lifting? And as I move to shallower water, how...
  11. B

    B Can You Use a Rubber Bracelet as a Buoyancy Reference for Freediving?

    First of all, I'm new to the forum, and rather uneducated, so I apologize this is likely in the wrong section. I've recently (like last night lol) started looking into freediving. I haven't taken any classes or anything yet, so all of this might be common and answered there. However, one of...
  12. F

    Modeling a kongming lantern (sky lantern)

    The problem gives these data external (cold) air temperature: ##T_c = 20^\circ{\rm C}## internal (hot) air temperature: ##T_h = 80^\circ{\rm C}## lantern volume ##V=0.2 {\rm m^3}## lantern surface ##A=2.0 {\rm m^2}## lantern mass ##m_{\rm L}=30 {\rm g}## external convective heat transfer...
  13. SpectraPhy09

    Buoyancy: Will the Object float or sink or remain at a depth?

    My answer was coming b but it's given wrong in my textbook. Because if a body is submerged completely in a liquid then buoyancy Force = mg So if the net force is Zero then shouldn't the body remain there? Plz can someone tell where did i went wrong? Also can a body float inside a liquid Because...
  14. M

    Calculating the Density of Kr: A Guide

    Hello. Firstly, I've calculated the density of Kr ( = 3.74 g/dm3), and I know that the p (fluid) = ρ * h * g. And then I've used the following equation: p1*V1 = p2*V2, and therefore: p1*V1 = ρ * h * g * (m/ρ) => p1*V1 = h * g * m. (h = 3.0153 m) Is that correct? Please, how could I calculate...
  15. B

    B When do marbles behave like a liquid in a beaker?

    If we place a cork in a beaker then filled the beaker with normal-sized marbles, the cork will remain stationary at the bottom of the beaker. However, if we continuously decrease the size of the marble, at a certain point, the marbles will behave like a liquid and the cork will rise to the top...
  16. 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...
  17. S

    I What are the effects of Buoyancy & G-force combined in this scenario?

    Hi all, I am trying to get it straight in my head how the interactions would work with a person (or accelerometer, for simplicity) suspended in a fluid, which is itself in a capsule which is then accelerated. Let's say with a rocket, on a linear track, to avoid any circular motion dynamics...
  18. kuruman

    B Upside down buoyancy (Have you seen this?)

    Devilishly clever.
  19. K

    Buoyancy force when a ramp is under water

    So I have made force diagram And I think that I should find the acceleration by using these equations: ##\sum Fx=w\sin(15)-f_k-T_{x-buoyancy} ## ##\sum F_y=N+T_{y-buouancy}-w ## I know that the volume of water displaced must be ##V=\frac{1}{2400}m^3## and the mass of the water is then...
  20. anuttarasammyak

    B Does Buoyancy Apply When Not Fully Submerged?

    I am discussing with friends whether concept of buoyancy is applicable even if the full surface of body is not covered by fluid(s). As an example a clay ball is dropping in the pool water. Buoyancy is working on the ball. Then it reaches the pool flat and sticks with it to make a mountain...
  21. Joe3502

    Buoyancy with the Cross-Sectional Area of a Rectangle

    Hi all, My teacher assigned us a problem to do a few days ago and have attempted it many times, often leaving and coming back to see if I could figure it out. I imagine that you would take the cross-sectional area and multiply it by how far under the surface of the water the rectangular object...
  22. 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...
  23. A

    I'm Confused about Buoyancy (Anime: A Certain Scientific Railgun)

    well I was watching the Third Season, and I was Confused, there's an esper her ability name is Float Dial it is an ability to control buoyancy. at first I understand, that it controls fluids thus able to slide on water walk on walker. but how the does she able to lift the heavy concrete and...
  24. brotherbobby

    Weight of water with objects floating

    We understand that the crucial thing about the problem is that the volume of water present in the three containers are not the same. Also, we note that in each case the weight of the container is the total weight of its contents. (A student might be confused as to why should be so - after all...
  25. T

    Buoyancy and the Archimedes Principle

    Hello all I was hoping someone could help me with some intuition regarding Buoyancy and Archimedes Principle. I thought i understood Buoyancy in terms of pressure but when i came across Archimedes Principle I realized I did not fully understand what Buoyancy is. I have an object that is a...
  26. men5j2s

    Buoyancy - (helium in a sealed vessel)

    Hi All, I'm trying to answer a question once and for all that has caused more debate than it ever should have (talking about an internal debate)... If I fill a sealed vessel (say an aluminium vessel with appox. 17.5ml of internal volume) with varying amounts of helium ( 1bar, 2bar, 3bar ...
  27. N

    Blocks Floating in Liquids

    Since they're all in the same liquid I'm assuming the buoyant forces would be the same on each block. But then I think about the volumes of the blocks, and them being different. I'm not sure if the block's volume would affect the buoyant force. Any help would be great, thanks!
  28. R

    Would a longer lever require less force to hand pump water? (PHOTO)

    I think that it would take less force to pump water based on the lever principle. But I'm not so sure! Here's a drawn photo.
  29. R

    Friction of Water Pumped Into 6in Tube from Chamber

    This is a deep well & the dimensions of the bottom chamber is 5x1x5ft which holds 187 gallons. The pump is located in this chamber with a piston/plunger only going up to the top of this chamber at 5ft. The pump pushes 187 gallons into a 6inch diameter pipe which is 1000ft long = 1,470 gallons...
  30. S

    B What happens to buoyancy when a non-uniform force is applied to the liquid?

    This is a thought experiment which I have had rattling around in my head for a while, and I think that it is too specific to find any answers for by google-fu alone, so I'm hoping someone here can help me to understand this! I have a fair understanding of buoyancy. The medium in which an object...
  31. J

    Hot air balloon buoyancy problem

    Initially the hot air balloon is stationary so... FB=Fg ρgV=mg m=1319.2...kg FB=Fg=12941N In the air... a=2d/t^2=0.933...m/s^2 Fnet=FB-Fg (1319-x)(0.93...)=12941-(1319-x)g x=114.6 kg but apparently this is wrong?...
  32. T

    How much sea water is needed to keep a submarine still at 30 meters depth?

    Homework Statement A submarine is in water, depth 30 meters. Inside submarine there is default air pressure. Submarine has volume of 125 m³, from which 10 m³ is water tank used for submerging. Submarine weights 123 tons. How large portion of the water tank has to be filled with sea water in...
  33. T

    Buoyancy, incompressibility and G forces

    Based on the Comex Hydra 10 tests, a diver breathing the correct mix can withstand a depth of 2300 ft, or the pressure of a column of water 2300 feet tall - nearly 1000 psi. Since the diver is buoyant (being the same average density as the water), they are essentially weightless. Not in...
  34. flamephoenix15

    How would you calculate how deep an object would go in water?

    Homework Statement So given a ball, radius r, mass m ,an a known height y from the surface the ball is dropped from, how would you calculate the depth the ball goes to in water (including the water it displaces), with the density of the ball less than the density of water. Ignore surface...
  35. E

    Solving a Buoyancy Problem: Cube of Ice in Water & Ethyl Alcohol

    Homework Statement A cube of ice whose edge is 17.0 mm is floating in a glass of ice-cold water with one of its faces parallel to the water surface. Ice-cold ethyl alcohol is gently poured onto the water surface to form a layer 5.00 mm thick above the water. When the ice cube attains...
  36. 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...
  37. Akash47

    Finding the density of a block in two layers of liquids

    Homework Statement A layer of liquid with density ##800~kg/m^3## floats on top of a volume of water. A block floats at the oil-water interface with ##3/4## of it in water and the rest of it in the liquid. What is the density of the block? Homework Equations Buoyancy =##vρg## where ##v##=...
  38. 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...
  39. Ripcord96

    How to approach the design for a boat step by step?

    Two pontoon boat design approach and how to find center of gravity, center of buoyancy, buoyancy, volume of displacement, meta center, meta centric height. I need to know is there a different approach to designing dual hulled boat than a single hulled boat and all the formulas and examples I...
  40. Why do whole oranges float, but peeled oranges sink {Buoyancy}. - YouTube

    Why do whole oranges float, but peeled oranges sink {Buoyancy}. - YouTube

    Video inspired by my wife ...ILY HADASA. When i asked her "Between a whole orange and a peeled orange which will sink ?" CORRECTIONS - None yet. Let me know ...
  41. YoungPhysicist

    Plumbing Build a Buoyancy Device: Ideas for Suspending in Water

    I want to make a device that can suspend in water via manipulating its density to as close to 1 as possible. I just thought that it will be cool to have a device that can stay in whatever position that I place it in the water. Any idea of how this could actually be done? I was thinking about an...
  42. chemisthypnos

    Does Increased Pressure Affect Buoyancy?

    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...
  43. W

    My Second Post: Change in Magnitude of Normal Force?

    Hello, Let us imagine a solid immersed in a liquid in a container such that the density of the liquid is less than the density of the solid. This means that the solid must sink. Let us study the solid when it reaches the bottom surface and is now at rest. The forces acting on the solid when it...
  44. M

    Buoyancy of Cube Question

    Homework Statement A large sealed container full of air falls into the ocean. It has internal dimensions of 6.06 2.59 2.43 m, with walls made of steel (8050 kg m3 ) which is 2.00 cm thick. Does it float? Homework Equations FBuoyancy = density*volume*g weight = mg m = density*volume density of...
  45. A

    Aluminium weight buoyancy problem

    An aluminium weight (B) is placed on top of a 0.472 kg wooden block (A) that is floating in water as shown below. The wooden block has a density of 533 kg.m-3 (3 s.f.). Calculate the required mass of the Aluminium weight to just fully submerge the wooden block as shown in the diagram. The...
  46. B

    Submarine Buoyancy Differential Equation

    Homework Statement A submarine of mass 80 000 kg is floating at rest (neutrally buoyant) at a depth of 200 m in sea water. It starts pumping out sea water from its ballast tanks at a rate of 600 litres per minute, thus affecting both its mass and the buoyancy force. Determine the vertical...
  47. 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...
  48. Z

    Upward force on each anchorage

    Homework Statement A steel pipeline carrying gas has an internal diameter of 120 cm and an external diameter of 125 cm. It is laid across the bed of a river, completely immersed in water and is anchored at intervals of 3 m along its length. Calculate the buoyancy force per meter run. Upward...