# What is Buoyancy force: 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. ### How to calculate the velocity of water streaming into the boat?

The problem: Its solution: I am sure that the velocity ##v## can be calculated by using Bernoulli's equation, but I fail to calculate it. I don't know what pressure is at the hole - is it ##h_2 \rho g## or ##(h_2 - h_1) \rho g##? The pressure of the water "under" the hole is ##h_2 \rho g##...
2. ### What helium mass in a balloon to make it bouyant in air?

I suppose that the temperature is the same for the helium as well as for the air. So ##\frac 43 \pi r^3 \rho g >m_{He}g+4\pi r^2 \sigma g##. I would determine the density of air from ##p_0 \mathrm d V=\frac{\rho \mathrm d V}{M_a}RT##. So ##m_{He}<4\pi r^2\left(\frac{p_0M_a r}{3RT}-\sigma...
3. ### Calculate the bouyancy of two spheres joined by a rope submerged in seawater

m * g = mAl * g V * ρ * g = VAl * ρAl * g V * ρ * g = V * ρAl * g ρ = ρAl this does not work at all, because the upper ball must have a density smaller than that of seawater 1200kg/m3 or not?
4. ### 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...
5. ### 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 ...
6. ### How Do You Calculate the New Density of Kr in a Balloon?

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...
7. ### Work done on moving a submerged object

I first worked out the buoyant force using rho = 1000, g = 9.8 and V = 1. (this gives Fb = 9800) I then worked out Fg of the tank (5 x 9.8 = 49) Then I used Fnet = Fb - Fg - T = 0 to work out tension as 9751. I thought the work done would be the work done by tension (9751 x 50.4), but I'm not...
8. ### 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...
9. ### Iron block dropped into a container of water sitting on a scale

Does the scale change during the block sinks? I am so confused... Is there any difference on change of scale with the applying of drag force?
10. ### 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.
11. ### 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...
12. ### 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...
13. ### Thrust on an triagular lamina emersed in a liquid.

I feel there is a really obvious flaw in my logic but i can't pin it down So i have to find the thrust on the lamina which is basically force of bauyancy F(bouyancy)= Vρg Now volume of the triangular lamina is its rea into its hieght. v = Ah hence F = Ahρgsome information i feel i didnt take...
14. ### Buouyancy force and Archimedes' principle

How much more will the volume of a fishingboat go under water, if I load the boat with 3.0m^3 fish with the density 0.90kg/dm^3? Fish : 3.0m^3 Density of fish: 0.90kg/dm^3? Homework Equations Archimedes principle: density * volume * g[/B]The Attempt at a Solution Tried setting upward force...
15. ### 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...
16. ### 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...
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. ### Is it correct that spherical buoy doesn't rotate?(Buoyancy)

Does spherical buoy doesn't rotate when subject to wave compare to rectangular buoy? If only buoyancy is taken into account only. Here is the picture I draw trying to explain. My reason is that the water that is touching the spherical buoy experience the same geometry on any surface of the sphere.
19. ### Is Newton's 3rd law sufficient to explain buoyancy force?

I assigned a question: Explain the physics mechanism that results in the buoyancy force. Some students replied: The floating object is interacting with the water. Therefore it exerts a downward force on the water. The water then exerts an upward force on the object due to Newton's Third Law...
20. ### 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...
21. ### Should I Use the Total Length of an Object to Calculate Buoyancy?

this is a float submerged in fluid the upthrust is greater than the weight. P=ρgh to calculate the pressure should I use the total length of this object as h? Homework Equations P=ρgh Fb=PA (A=area) The Attempt at a Solution Curious to know whether I should use the total length of this object...
22. ### 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...
23. ### Optimizing Mass for Vertical Rod Buoyancy in Unknown Fluid

Homework Statement A 0.75-m rod has a uniform linear mass density of λ. A small mass m with negligible volume is attached to one end of the rod. The rod with the attached mass is placed in a container of unknown ﬂuid and after oscillating brieﬂy, comes to rest at its equilibrium position. At...
24. ### Container of Liquid Accelerated Upward

Homework Statement A block floats partially submerged in a container of liquid. When the entire container is accelerated upward, which of the following happens? Assume that both the liquid and the block are incompressible. A) The block descends down lower into the liquid. B) The block does...
25. ### How Does Temperature Affect the Lifting Capacity of a Hot Air Balloon?

Homework Statement A hot-air balloon stays aﬂoat because hot air at atmospheric pressure is less dense than cooler air at the same pressure.If the volume of the balloon is 500 m3 and the surrounding air is at 60◦F. What is the maximum load (including the weight of balloon, but excluding the...
26. ### 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.
27. ### Simpson's rule with changing volumes

Hi there, Homework Statement I am doing a simulation of a free-fall lifeboat trajectory into the ocean and there is a part where i need to get the volume of the lifeboat to obtain its buoyancy force. Buoyancy force is proportional to the immersed volume of the body and this volume can be...
28. ### Optimizing ROV sinking velocity using buoyancy and resistive force

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...
29. ### Calculating Specific Gravity and Density of a Body

Homework Statement If you have a body that weighs 100 N dipped in oil that has density equal to 0.8 g / cm³. This same body when immersed in water begins to weigh 60 N. How do I find the density / specific body mass? Homework Equations Solution= 1,3 g/cm³ The Attempt at a Solution Already...
30. ### Calculating Density/Specific Body Mass: Need Help?

If you have a body that weighs 200 N dipped in oil that has density equal to 0.8 g / cm³. This same body when immersed in water begins to weigh 60 N. How do I find the density / specific body mass? Already tried using the formula of the apparent weight = actual weight - buoyant and not worked...
31. ### Understanding Archimedes' Principle: Solving for Density in a Buoyancy Problem

Homework Statement If an object is 500 N normally but has an apparent weight of 300 N when submerged in water, what is the density of the object? Homework Equations ρobjVobjg = ρfluidVobjg The Attempt at a Solution So the correct answer is 2500 kg/m3 but when I set up the problem, it doesn't...
32. ### Why is Buoyancy Force = to the density of the liquid * Vobj(submerg)*g

Why is the weight of the liquid displaced = buoyancy force and not the weight of the submerged volume of the object? Does it have something to do with normal force of the surface of the liquid on the object or pressure?
33. ### Buoyancy force on object problem

A object B is place on top object B and the object B is fully submerged in the water. So is the bouyance force equal the force apply by object A on object B? Please help.
34. ### Buoyancy force and surface tension

The buoyancy force is defined as the difference in pressures on the top and bottom of an object submerged in a fluid. When an object is pushing on the top surface of a fluid from underneath, the force of surface tension begins to act downward on the object, preventing it from breaking the...
35. ### How to Approach Buoyancy Force in Fluid Mechanics Homework?

Homework Statement I've attached the problem I'm kind of lost as to how to even start this problem. For part ) I think my first approach is to sum up forces in the vertical direction. Which would yield F-W1-W2=ma=0 Where: F=buoyancy force W1=weight of submerged volume of oak...
36. ### Buoyancy force on the partially submerged object

Archimedes's principle says that the buoyancy force would be equal to the weight of the fluid displaced by the body. It's easy to work with the completely submerged object since I just need the whole volume of the object to as it is equal to the volume of displaced fluid. However, if an object...
37. ### Buoyancy force of a hot-air balloon

Homework Statement The envelope and basket of a hot-air balloon have combined mass of 331 kg. The spherical envelope of the balloon has a radius of 10 m when fully inflated. What is the maximum mass of the passengers the balloon can carry when the temperature of the gas is 108 o C? Use...
38. ### Buoyant Force and Displacement Volume of a Boat in Lake Superior

Homework Statement A boat full of people lazily drifts along in Lake Superior. The boat has a mass, including people of 15,450kg. A. What is the buoyant force? B. What is the volume of the displaced liquid in cubic meters? The Attempt at a Solution A. Fb=mg Fb=? m=15,450kg g=...
39. ### A Question About Buoyancy Force (I Think My Teacher Is Wrong)

How can they be equal at that point? And where is the force that the string applies. By the way Fk = Buoyancy Force
40. ### Buoyancy force calculation question

Hi, I'd like to calculate the buoyancy force exerted on a gate in an intake channel (see picture in attachment). Since the gate is not totally surrounded by water, I guess that the normal buoyancy formula doesn't apply here, am I right ? Is there a rule of thumb for this kind of problem ...
41. ### Pressure distribution of buoyancy force

Let's say we have a solid cylinder which is fully submerged in water or any fluid having density lower than of cylinder's density. I want to find out how can we convert the buoyancy force in terms of pressure. Actually I am doing FE analysis of this kind of problem and software allows to apply...
42. ### Calculating Helium Balloon Buoyancy Force at Launch and 2km Altitude

Homework Statement A helium meteorological balloon is made of a bag of impervious fabric that does not stretch, and when fully inflated forms a spherical shell of 1m diameter enclosing the He. At launch it is filled with He (at STP) to 15% capacity. The launch takes place in the Antarctic, in...
43. ### Buoyancy Force question, having hard time starting.

Homework Statement The 3m plank shown in section has a density of 800 kg/m3 and is hinged about a horizontal axis though its upper edge O. Calculate the angle theta assumed by the plank with the horizontal for the level of fresh water shown. Homework Equations Whats wrong with the editor...
44. ### Is there a buoyancy force or not?

Hi all, If i attach a cylinder at the bottom of the water tank such that some of its part is out out of water. So in this case what would be the buoyancy force acting on the cylinder? -agtee.
45. ### Buoyancy Force and Block Equilibrium

Hi, A cylindrical log with volume 2.0m^3, length 4.0m and density 700 kg/m^3 is anchored to the sea floor by a light cable attached to one end, and with the long axis vertical. Density of sea water is 1030 kg/m^3. g=9.80 m/s^2. a) What is the buoyancy force acting on the wood? I did...
46. ### What Minimum Height Must the Sides of a Steel Boat Have to Float?

Homework Statement The bottom of a steel "boat" is a 7.00 m x 8.00 m x 5.00 cm piece of steel (rho-steel=7900 kg/m^3.) The sides are made of 0.540 cm-thick steel. What minimum height must the sides have for this boat to float in perfectly calm water The Attempt at a Solution I...
47. ### How Does Buoyancy Affect a Bubble's Force as It Sinks?

First off, thanks in advance to anyone who can help at all. I've attempted to do a few things, and none of them seem to work...let me know what you think! THANK YOU! Homework Statement A sperical bubble of radius r has a volume v and a pressure p when just below the surface of a liquid...
48. ### Calculating Buoyancy Force of a Cuboid: Exploring Water Displacement

Hi, This is my first post here so please be gentle! I am looking to investigate the buoyancy force of a cuboid. The dimensions of the cuboid are 30cm x 20cm x 15 cm. When the cuboid is placed in the water (the face 30cm x 20cm faces the water), 4cm of the cuboid will be submerged. If I push the...
49. ### Finding Buoyant Force of Air: Calculating Volume Displaced

How do you find the exact buoyant force of air? B = pgV where p = density of air (1.29kg/m3) g = 9.81 m/s2 V = volume displaced The problem is, you can't measure Volume directly. You have to do it experimentally by weighing a mass in air and then in water. I can...
50. ### What is the direction of the buoyancy force?

What is the direction of the buoyancy force? Why the buoyancy force acts in this direction?? Thanks for any help:smile: