Buoyancy force Definition and 21 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. M

    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...
  2. M

    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...
  3. Richard Spiteri

    B Buoyancy and External Forces

    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...
  4. K

    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?
  5. Richard Spiteri

    I Neutrally Buoyant Bodies

    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.
  6. 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...
  7. Navin

    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 = Ahρg some information i feel i didnt...
  8. L

    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...
  9. YoungPhysicist

    Plumbing A buoyancy device

    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...
  10. 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...
  11. K

    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.
  12. 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...
  13. WherE mE weeD

    Calculating upthrust of float

    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...
  14. Z

    Buoyant Force Question

    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 G...
  15. C

    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...
  16. Ian Baughman

    Physics 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...
  17. Ravi Singh choudhary

    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.
  18. N

    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...
  19. vincekillics

    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...
  20. Poquinha

    Buoyancy Force

    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...
  21. Poquinha

    Buoyancy Force

    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...