# What is Bouyancy: 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. ### Solving the Mystery of a Floating Bottle: Density & Volume

If we throw empty but sealed glass bottle in water it will float with 60% of it's volume above the surface of water. What is average density of floating bottle and what is it's volume? Mass of bottle is 0.4 kg(mass of air inside of bottle is irelevant). It's easy problem but I can't get right...
2. ### Density versus Pressure -- Balancing the bouyancy of a floating capsule

[Mentors’ note: no template as this thread had initially been misplaced in the technical forums and was moved here] Summary:: Enclosed Cubic foot capsule passing between two bodies of different densities but questioned pressure. You have a tub of fresh water 32 feet high sharing a wall with...
3. ### B Buoyancy thought experiment

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...
4. ### Bouyancy Question: Two weights on strings, with one weight submerged

Hi new to the physics world and the symbiology is hard to understand completely. Attached is the work I've done to a final solution but I can't seem to get the answer in terms of only density of water and T1 and T2. Thank you for any assistance. [ Mentor Note -- Word file replaced with a...
5. ### Bouyancy force -- What mass can the balloon carry?

In a I used FBouyancy - FBalloon -F mass = 0 rewrote mass as a product of density and volume to obtain; m= v(densityair - densityHe) m= 1031.5kg.I am stuck in part b. I have this formula called Barometric formula which ı should use; P = P0 * e((-P0*g*h)/ P0 So what I understood is that...
6. ### What's the difference between hydrostatic force and bouyancy?

Homework Statement Homework EquationsThe Attempt at a Solution
7. ### Why do bubbles of air in water move up?

What I am confused about is why do bubbles of air in water move up. I understand why solids and liquids would move up in water if they are less dense. I get the idea that the deeper you go in water the more pressure there is because of the more water weighing down on the water and so there would...
8. ### I Could tide-powered buoyancy be a viable source of energy?

Hi all, this is my first post. It seems around eight years ago I was on a similar brain-wave-length as Laurence Kemball-Cook, with his idea to create a power generating flooring surface... only he had the know-how to bring the idea to market, so credit where credit is due. (though now I wonder...
9. C

### I Buoyancy of Helium and Hydrogen Balloons: Calculations and STP Considerations

As a note, this is my first post on this site. I was looking at some bouyancy calculations with regard to balloons being in air when I came across this post. I cannot understand why the calculation for the net force of bouyancy done by Vintage (the last post on the page) does not include the...
10. ### Difficult Bouyancy Question

Homework Statement Homework Equations F = ma F = pVg p = m/V 3. The Attempt at a Solution [/B] First i tried finding the volume of displaced water which i did by... (50/1000) * 10 = 1000 * 10 * V ---> V = 0.00005 m^3 Next as the metal sits on the bottom its weight force minus the bouyant...
11. ### B Bouyancy: mass and displacement

This is something I've always wondered and I have never taught about this at school so I leave a thread on this forum. I know that displacement = the mass of the object, but I don't quite get this concept. For example, if you have a kilogram of gold and a kilogram of iron and put them in...
12. ### 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...
13. ### What would happen to the ball?

Homework Statement Assume that the beaker is half filled with Hg, the steel ball being lighter will float in it half-submerged. Now we pour water into the beaker, so that the beaker is completely filled with water on top, Hg in the bottom and steel ball floating between them. Now my question...
14. ### Fundamental frequency change because of bouyant force

Homework Statement A rope has an end fixed and the other is passing through a pulley and has a body attached to it. The fondamental frequency of the rope is initially ##f_1=400 Hz##. If the body is then put in water the fondamental frequency of the rope becomes ##f_2=345 Hz##. If the linear...
15. ### Floating block with objects that are thrown in a liquid

Homework Statement I'm confused about the following kind of situation. Consider a block of density ##\rho_b##, mass ##M_b## and section ##S_b## that floats on a liquid of density ##\rho_l##, in a tank of section ##\mathcal{S}##. On the block there are some objects (all equal), of density...
16. ### Density floating problem, wood and lead

1. Homework Statement A block of wood has a mass of 3.47 kg and a density of 628 kg/m3. It is to be loaded with lead (1.13 × 104 kg/m3) so that it will float in water with 0.848 of its volume submerged. What mass of lead is needed if the lead is attached to (a) the top of the wood and (b) the...
17. ### I Force applied to a fluid

If you place an solid object on a fluid, is the force it applies to the fluid equal to the buoyant force? I would have thought that it applies a force equal to its own weight but that would mean that the force applied to the object by the fluid, the buoyant force, would not be equal and opposite...
18. ### Buoyancy With Solid Sphere

A small solid sphere of mass M0, of radius R0, and of uniform density ρ0 is placed in a large bowl containing water. It floats and the level of the water in the dish is L. Given the information below, determine the possible effects on the water level L, (R-Rises, F-Falls, U-Unchanged), when that...
19. ### Bouyancy, Archimedes principle

Homework Statement I have confusion. Suppose i have metal cube placed in empty vessel. I have seal tight the base of cube so that the water or any fluid can't seep below the cube. Now i pour water into the vessel. Will there be bouyancy on the metal cube ?? If i measure the weight of cube, will...
20. ### Solve Bouyancy Question: Max People in Crate Before Sinking

Homework Statement A large number of people are on a sinking ship, and decide to use a large crate with an open top as a lifeboat. A physicist in the crowd has been tasked with finding how many people can board the craft before it would sink. She assumes the average person has a mass of 70kg...
21. ### Centrifuge and Radial Bouyancy in space

Hi all, I want to calculate how tell if a particle in a centrifuge in deep space will move towards the rotation axis or to the rim of the fluid. There is no container walls. This is not a HW problem. I watched the video and wanted to learn more about this. (it is under the you-tube...
22. ### Bouyancy and liquid separation

Would it be true to say that boyancy, as in helium rising and liquids of different densities separating, with the densest at the lowest point, is an indication of gravity's natural tendency to attract the most dense mater to its centre point? If so is the Schwarzschild radius a natural...
23. ### Preparing for a phys final, springs and bouyancy

Hey all, I am a little confused on this, I was trying to study and these two questions came up... I guess i am confused, I don't know the solutions and I want to know which it is and why... 1. A helium balloon is observed to rise up into the air at the front of the classroom, and is only...
24. ### Tricky density and bouyancy question

Homework Statement 1. A piece of metal weighs 53.1 N in air, 36.6 N in water, and 41.7 N in an unknown liquid. Find the density of the metal. Answer in units of kg/m3 2 Find the density of the unknown liquid. Answer in units of kg/m3 2. Homework Equations [/b] p= M/V W= mg...
25. ### Bouyancy, capillary action. HELP PLEASE

Homework Statement so i have an to do an EEI for an assignment due this friday. My experiment involves me putting a rounded bowl with a flat bottom and a hole in the center on the bottom onto some still water and testing how the bowl sinks faster as weight is added to the bowl. The water comes...
26. ### Bouyancy of a submarine

Bouyancy Question: A 3000m^3 submarine of mass, m, is filled with air(density air =1.29kg/m^3) and is floating, submergerd, in seawater(density = 1400kg/m^3) a) draw diagram. b) Write an equation that that equates weight of the sub to the buoyant force( include weight of air)? c)...
27. ### Calc # of Persons Carried by Hydrogen Balloon

Homework Statement Determine the no. of persons of 80kg=mass that can be carried bya a 20-meter diameter balloon. filled with hydrogen gas. at 5° c and 101 K pa|a| if the atmospheric pressure is 101.325 kpa|a| at 10° c assume g= 9.795 m/s² can you help me figure this out?
28. ### Newtons from Bouyancy underwater comparing helium, oxygen, and hydrogen

I have an under water application where I need the maximum amount of buoyancy with a limit to the volume i can use for a lifting device. My question is how much more Newtons would be exerted for helium instead of oxygen, or hydrogen instead of oxygen underwater? What is the percentage...
29. ### Interesting bouyancy theoretical question

Hello, I have a question Lets say you have a container of water that weighs 10 pounds including the water/container material and u have a 5 pound object that can float quite easily in that water and u attach a piece of string to that object to the bottom of the container of water to the...
30. ### Tough Question: Bouyancy and 1-D Motion

Homework Statement A ball whose density is 0.4*103 kg/m3 falls into water from a height of 9cm. To what depth does the ball sink? Only consider bouyancy and ignore retardation due to viscocity. (a) 9cm (b) 6cm (c) 4.5cm (d) 2.25cm Homework Equations Bouyant force =...
31. ### Beaker of water and bouyancy in a sink

Homework Statement You place a glass beaker, partially filled with water, in a sink. The beaker has a mass of 517 g and an interior volume of 280 cm3. You now start to fill the sink with water and find, by experiment, that if the beaker is less than half full, it will float; but if it is...
32. ### Calculate Weight of 3m^3 Ichthyosaurus in Metric Tonnes

Homework Statement Fossilized ichthyosaurus remains enable us to estimate their volume. If we assume that a 3m^3 icthyosaurus was "wightless" when it swam under water. What was its weight in metric tonnes? plugging this into the equation would give me B .. not weight...
33. ### Calculate Weight of "Weightless" Ichthyosaurus in Metric Tonnes

Homework Statement Fossilized ichthyosaurus remains enable us to estimate their volume. If we assume that a 3m^3 icthyosaurus was "wightless" when it swam under water. What was its weight in metric tonnes? Homework Equations B=Wdisp. lqd ?? The Attempt at a Solution B=V x...
34. ### Solve Bouyancy Problem: Homework Statement & Equations

Homework Statement You stand at the edge of a pool of water with a rock of mass M in your hand. You throw the rock up in the air. The rock leaves your hand at a height h0 and rises to a maximum height h1 before falling into the water. After entering the water, the rock eventually submerges...
35. ### Terminal Velocity and Bouyancy - F = kv

Homework Statement A rock of mass 0.400 kg is released from the surface and sinks in the ocean. As the rock descends it is acted upon by three forces: gravity, buoyancy, and drag. The buoyancy is an upward force equal to half its weight. Drag from the water can be modeled by F = kv...
36. ### Fluid Mechanics - Bouyancy Question

Homework Statement A block of wood floats on water. The density of the wood is .634g/cm^3. Its mass is 243 g. Find the minimum mass (g) of lead we must attach to the block so that it will sink, along with the lead. The density of lead is 11.3 g/cm^3. Caution: you must take into account...
37. ### Bouyancy of Ice in Water

Homework Statement Two Identical glasses are filled to the same level. One is filled with fresh water (density=1 gm/cm^3) and the other with salt water (density=1.025g/cm^3) Into each glass a cube of ice (density= 0.92 gm/cm^3) one cm on each side is placed. What is the height of the ice above...
38. ### F=MA but including Bouyancy

Homework Statement Before you submerge a block (25N) in water, you place the container of water on a scale. The scale reads 100N. You then immerse the block in the water, the block is also hung on a scale that reads 17N. Is the scale reading greater than, less than, or equal to 100N? (...
39. ### Weight & Buoyancy: Will the Measured Weight Be Less?

Homework Statement A block of wood floats in a beaker of water. According to Archimedes' principle, the block experiences, an upward bouyant force. If the beaker wit the water and floating block were weighted, would the measured weight be less than the sum of the weights of the individual...
40. ### Question regarding Bouyancy in Water

This has been bugging me for a bit. Take two, identical, rigid containers. Fill one with helium at the level in say... a balloon? Then take the other one, and suck all the air out. Take them to the bottom of a container of water- Which will go up quicker?
41. ### Ice Cube Floating: Help with Bouyancy Question!

Hi! I'm new to the forums and this is the first that I'm posting something :D! I have a quick question from my textbook that I got wrong. Can someone please help me out! Thanks in advance! Question: An ice cube is floating in a glass of water that is filled entirely to the brim. When the...
42. ### Solve for Ratio of Steel Ax and Aluminum Piston Weights - Buoyancy Problem

[SOLVED] bouyancy problem Homework Statement If a steel ax and a aluminum piston have the same apparent weights in the water which can be written as (Pax)(Vax) - (Pw)(Vax) = (Pp)(Vp) - (Pw)(Vp) where P=rho(density), p=piston, V=volume, and w=water Find the ratio (Wax/Wp) of their weights...
43. ### Weighing a ship by bouyancy

so your company has built an enormous ship and it is being lowered into a large pool to test if it can float... another company has contacted you to buy your ship and they want to know how much the ship weighs... your company has spent all its money building the ship and you can't afford a...
44. ### Bouyancy, Pressure, Spring Scale

1. A 11.4kg block of metal is suspended from a scale and immersed in water. The timensions of the block are 12.7cm x 9.9cm x 9.2cm. The 12.7cm dimension is vertical, and the top of the block is 4.94cm below the surface of the water. The acceleration due to gravity is 9.8m/s^2. What is...
45. ### Need Help finding radius of ball using bouyancy

Homework Statement An iron ball is suspended by a thread of negligible mass from an upright cylinder that floats partially submerged in water. The cylinder has a height of 6.00cm, a face area A=12.0 cm^2 on the top and bottom, and a density \rho_c=.300\frac{g}{cm^3}, and 2.00 cm of its height...
46. ### How can I design an experiment to test bouyancy for my physics EEI?

This if my first post and I think this is the right place to post this, hope so anyway. Just wondering if you guys had any ideas for a physics experiment relationg to bouyancy. I have to do a EEI (extended experimental investigation) for physics and so far I don't have too many ideas. I can...
47. ### Deep Glider, depth, and bouyancy

I was fascinated by the Wired article describing such an elegant engineering solution for remote data collection deep in the ocean, Deep Glider: http://www.wired.com/news/technology/0,72794-0.html?tw=rss.index" [Broken] But, having been out of engineering school a few decades and a bit rusty...
48. ### Re-Post of a Bouyancy Problem

Previous Thread https://www.physicsforums.com/showthread.php?t=55809 Problem, again.. The bottom of a steel "boat" is a 7.00 m\times 9.00 m\times 5.00 cm piece of steel (Psteel=7900). The sides are made of 0.540 cm-thick steel. What minimum height must the sides have for this boat to...
49. ### Air volume, compression and bouyancy

Hi everybody! I'm new to this forum and i have a question i hope someone of u can answer... If i have a cylinder (made of plastic or some other light weight material) 1 meter long with an area of 1 dm2 that would be 10 liters right? Now, if i place the cylinder vertical and place a 8 kg...
50. ### Bouyancy - throwing rock overboard

k in my advanced physics class 32S we are doing a project on buoyancy, one of the questions we have to answer is if you go out in a pool in a boat with a rock in it, then you throw the rock over board will the water level go up/down/stay the same. I am pretty sure that the water level will go...