# Buoyancy Definition and 69 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 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...
2. ### 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...
3. ### Calculate the depth to which a balloon full of Kr must be pushed underwater to make it sink to the bottom of the sea

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...
4. ### 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...
5. ### 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...
6. ### Will the water level rise?

(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...
7. ### 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...
8. ### 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!
9. ### 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.
10. ### Will the friction of water pumped into a smaller tube from large well be significant?

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...

12. ### 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...
13. ### 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##=...
14. ### 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...
15. ### 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...
16. ### 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 ...
17. ### 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...
18. ### 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...
19. ### 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...
20. ### 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...
21. ### 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...
22. ### Stability of a wedge problem

Homework Statement If a beam with square cross-section and very low density is placed in water, it will turn one pair of its long opposite faces horizontal. This orientation, however, becomes unstable as we increase its density. Find the critical density when this transition occurs. The density...
23. ### B Archimedes' Principle for gases - derivation?

Hi all, I understand where Archimedes' Principle comes from in liquids: If we imagine a cylinder immersed in a liquid of density ρ whose cross-sectional area is A and whose top is at depth h1 and whose bottom is at depth h2: Force(top of cylinder) FT = ρgh1A Force(bottom of cylinder) FB =...
24. ### B How things Float

Imagine a container containing water up to 100cm of its height then I dip a cube of plastic in the water on depth of 75cm. We all know it will float because of its less density, but if we go with Pascal Law, The pressure/force applied by the water above the cube is more than than the buoyant...
25. ### I Object suspended over an empty bucket

I was recently tutoring a first year student, and a question of her assignment was as follows: Suppose that that you have a bucket of water over a scale. If you then partially submerge an iron rod in the water, while holding the rod so that it does not touches the bucket, will the reading on...
26. ### Using Archimedes' principle in engineering applications

Homework Statement A cylindrical buoy floats in sea water with its axis vertical so that it's three-fourths submerged. The buoy is 0.8m in diameter and 2m in height. Its fabricated from iron plate 10mm thick. Calculate the mass of iron chain securing the buoy. The relative density of iron is...
27. S

### Buoyancy: Cube of Iron - solved this correctly?

Hi, Just wondering if I'm going about solving this problem correctly: "A block of iron quickly sinks in water, but ships constructed of iron float. A solid cube of iron 1.6 m on each side is made into sheets. From these sheets, to make a hollow cube that will not sink, what should the minimum...
28. ### B Is it possible to harvest energy from fluid buoyancy?

So I've noticed my little sister's helium balloons floating at the ceiling. Made me wonder if someone could create a device that could harvest energy from this buoyancy. What if I tied a very long rope to a huge balloon filled with helium, and rolled the other end of the rope to a gear attached...
29. ### Buoyancy question -- A raft with different loads....

Homework Statement [/B] A Raft with dimensions, 5m for length, 3m wide and 0.12m thick, is placed on water. The raft has a density of 320 kg/m^3. a) What volume of the raft is submerged into water, if the density of water is 1150 kg/m^3? b) A man and his elephant board the raft. How...
30. ### B About buoyancy and 2 equations

1) My teacher says that the apparent weight of an object in water (floating,sunken,submerged etc) is equal to it's actual weight-buoyant force acting on it. That is, wt (ap)=wt (ac)- F(b) Where wt (ap) = apparent weight, weight (ac)= actual weight. And f (b) = buoyant force 2) But..., if an...
31. ### Fluid Mechanics and Archimedes Principle

Homework Statement [/B] A rectangular object has a width of 40 meters, height of 15 meters, and length of 2 meters. It floats consistently when 3 meters of its height is below the surface of the water. 1. Find the volume of the displaced water. 2. How much is the buoyant force on the object...
32. ### B Rate of sinking objects

Hi All, I have a problem (with 3 separate instances) to which I believe I have the answers, but would like check with those more knowledgeable than myself. They revolve around 3 blocks sinking through water and which falls quicker. I am ignoring friction. Instance 1: All blocks are exactly the...
33. ### What will happen to the water level of the swimming pool?

if the person sitting in the boat throws a pebble to the swimming pool. Pebble was initially contained inside the boat and of course it has higher density than water.
34. ### Hot air balloon descent

If a hot air balloon cools enough to start descending, does it keep accelerating until it hits the ground? Assume that the air inside does not cool off anymore, and pretend that the air pressure stays the same at all altitudes. I tried testing it by dropping a coin into a pool. It didn't seem...
35. ### Buoyancy help

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...
36. ### Oil & water problem

How does a ball floating 50% in water move when a large amount of oil is added, and why? MY Solution: I think that the ball rises because when a large amount of oil is added, the oil sinks to the bottom causing the water to be pushed up, increasing the buoyant force which causes the ball to...
37. ### Terminal Velocity of a Cylinder Freefalling Through a Fluid

Hello :) I am an engineer and I am trying to analyse a system which basically contains a cylindrical body free-falling through a body of static water beginning with zero velocity. I am ultimately trying to find what the velocity of the object would be at a depth of 20m. In order to do this I...
38. ### Can drag force and lift force be in the same direction here:

In the attached picture, can I say that there is a lift force in the Y-direction, and a drag force too in the same Y-direction? FL proportional to V_fx2 ? FD proportional to V_py2 ? Is this equation of motion for the Y-direction correct here: ma = − mg − FD + V(rho)g + FL The lift force is...
39. ### Helium for flying-Buoyancy

Hello everyone, Following Buoyancy formula, how much helium is required to help someone with 80 kg weight against gravity? While the Buoyancy depends on volume, what happens if we compress helium? Thank you in advance. Regards, Behrouz
40. ### Finding tension in a string from a suspended object

Homework Statement A 0.02 m x 0.01 x 0.03 block of copper (density = 8.8 g/cm3) is suspended submerged in milk (density = 1.03 g.cm3) by a string. How much tension is in the string? Homework Equations The Attempt at a Solution My initial thought for an equation was: T+Fb(buoyancy)-mg=0 T+...
41. ### "Build the best boat" with given area

Hi. Given the area, what is the shape of an infinitely thin surface that can carry maximal load on water, i.e. has the best buoyancy just before water gets in? Is it the hemisphere?
42. ### Help with buoyancy homework?

Twenty per cent (20%) of a rubber ball is floating above the surface of a pool of water. If water has a density of 1000 kg/m^3 and the ball has a mass of 3 kilograms, what is the volume of the ball?
43. ### Velocity of sinking submarine

Homework Statement A small spherical under water ROV (remotely operated vehicle) has a radius of 0.5m and a mass of 450kg. It sinks or rises in the ocean by taking water on board or pumping it back out again. How much water must it take on board to sink at a constant velocity of 1.2m/s. The...
44. ### Archimedes' buoyancy question

Question: Balls A and B of equal mass are floating in a swimming pool, as shown below. Which will produce a greater buoyant force? (Image shows two circles with circle A larger than circle B) A. Ball A B. Ball B C. The forces will be equal D It is impossible to know without knowing the volume...
45. ### Finding volume of a submerged object

Homework Statement There is a block of wood floating on the surface of a body of water, with a ball attached to the bottom of the block by a string. I am asked to find the volume of the ball given the tension in the string. We also know the volume of the wood block from an earlier problem if...
46. ### Producing a shearing force diagram and bending moment curve

Homework Statement A box shaped vessel, of ##4## compartments and ##80m## lenght, light displacement of ##800## tonnes loads ##200## tonnes in the first compartment and ##200## tonnes in the last compartment. Produce the shearing force diagram and bending moment curve. Homework Equations 3...
47. ### Atmosphere layer

Dear PF Forum, Our atmosphere consist of 78% N2, 21% O2, 0.9% Argon, and other... https://en.wikipedia.org/wiki/Atmosphere_of_Earth What about theses gases? Will they form a layer like this liquid because of their buoyancy difference? Or they will be scattered evenly because of the wind. I...
48. ### A simple question about buoyancy.

Situation : A man faces difficulities to pull out a fish from the water when half of the fish body is already out of the water. How would you explain this?
49. ### 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...
50. ### Density of bar submerged in water

Homework Statement A bar suspended in air weighs 1.75 N The same bar weigs only 1.4 N when suspended in water. Calculate; a. upthrust b. density of the bar Homework Equations D = m/v F = m*a The Attempt at a Solution a[/B]. Upthrust is 1.75 N - 1.4 N = 0.35 N b. Assuming density of water...