What is Buoyant: Definition and 167 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

    Calculating Density of Oil Using Buoyant Force Formula | Homework Help

    Homework Statement Buoyant force. A block of wood floats in oil with 90.0% of its volume submerged. What is the density of oil? The density of the block of wood is 0.67 g/cm3. Homework Equations buoyant force = pgV The Attempt at a Solution p(oil) V(oil)=p(h20) V(h20)...
  2. P

    Buoyant force: wooden block in oil and water

    So there's a wooden block floating on water in such way that 78% of the block is submerged in water. Now oil (with density of 700 kg/m^3) is poured on top so that the wooden block is wholly covered by the oil. Now how much of the block is submerged in water? (Hint: do not ignore the buoyant...
  3. B

    How Many Logs Are Needed to Keep Children Afloat?

    Three children, each of weight 356 N, make a log raft by lashing together logs of diameter 0.30 m and length 1.80 m. How many logs will be needed to keep them afloat in fresh water? Take the density of the logs to be 800 kg/m^3. I'm interpreting this problem as a buoyancy problem for...
  4. A

    Calculating Buoyant Force on a Helium-Filled Balloon

    buoyant forces I was having trouble with this problem. please let me know how to approach this. thanks. An empty rubber balloon has a mass of 0.0135 kg. The balloon is filled with helium at a density of 0.181 kg/m3. At this density the balloon is spherical with a radius of 0.400 m. If the...
  5. O

    Not so simple buoyant forces in bubbling water

    Many of us have probably seen the TV documentary offering the theory that ships at sea in places like the Bermuda triangle might be sinking because methane gas released at the sea floor creates bubbles that lower the density of the water, resulting in reduced buoyancy. The theoretical...
  6. S

    How Do You Calculate the Lifting Capacity of a Helium Balloon?

    Say I have some volume V of some gas with a pensity P less than the density of air. How can I calculate how much a balloon (of volume V) of this gas could lift? Is this called the "buoyant" force, or is this something else? What I'd like to do is find the find the lifting "power" (...) of some...
  7. C

    Calculating Buoyant Force of Ice to Find Mass - No Hints

    70% of a mass is supported by a slab of ice and the ice sinks down so that only half of what was previously exposed now is exposed. What is the mass assuming that the ice has a volume of 10 m^3 and the mass has a specific gravity of 1.0? Why can't I use the buoyant force of the ice before and...
  8. K

    Solve Buoyant Force Problem: Helium Balloon for Levitation - 70.1 kg Mass

    Hi, I am having some trouble with the following question: You (mass 70.1 kg) decide to take off--literally--with a helium balloon of mass 31.6 kg. The densities of air and helium are air = 1.321 kg/m3 and helium = 0.179 kg/m3. What volume of helium is needed to levitate you and the balloon...
  9. B

    How do you find density when given the buoyant force

    Hey This has stumped me and 2 other people who have also tried to solve this problem. My instructor was about to explain it but just left us with we can get it through the equation of Buoyant force Bf=pgv. Here is the actual question from my book. I have no idea how your supposed to...
  10. B

    Buoyant Force and Volume: Doubling the Volume, Doubling the Displacement?

    I've already determined the first part of the question, which is regarding the buoyant force. An object weights 5N and displaces 3.5 N of water when placed in the beaker. The buoyant force is = to the displaced water which is 3.5N. Now, here's where I'm stuck. The volume doubles, and weight...
  11. E

    What is the buoyant force exerted on the balloon in air

    Is this a correct interpretation of Archimedes Principle for this application? A balloon is filled with helium (density = .02 kg/m^3) and has a volume of .009 m^3. What is the buoyant force exerted on the balloon in air (density = 1.3 km/m^3)? Fb = (1.3 km/m^3)x (.009 m^3) x (9.80 m)...
  12. M

    Calculate the buoyant force exerted by the water on the sphere

    A hollow, plastic sphere is held below the surface of a freshwater lake by a cord anchored to the bottom of the lake. The sphere has a volume of 0.700 M^3 and the tension in the cord is 760 N. Calculate the buoyant force exerted by the water on the sphere. Take the density of water to be 1000...
  13. R

    Calculating Payload for Helium and Hydrogen Balloons | Buoyant Forces Explained

    (a) A light balloon is filled with 371 m3 of helium. At 0°C, the balloon can lift a payload of what mass? (b) In Table 14.1, observe that the density of hydrogen is nearly half the density of helium. What load can the balloon lift if filled with hydrogen? I really have no clue where to...
  14. F

    Buoyant forces on hot-air balloon

    hey guys i have an exam tomorrow so i guess a uick reply would help .. my mind is totaly blocked as to how to solve this question. hot air baloon experiences an acceleration of 1.10m/s total mass equals 315kg a) what is the upward (buoyant) force on the system b) the baloonist wishes to...
  15. A

    Understanding Buoyant Force: Impact of Density on Objects Immersed in Water

    If two objects are the same shape and have the same dimensions, but one object has a bigger density, does that affect the buoyant force? Will the object with a bigger density have a greater buoyant force? They're both immersed in water. I say no because the buoyant force depends on the...
  16. holly

    Floating in Fresh & Salt Water: Buoyant Force & Wooden Boat Impact

    I'm really fired up about physics now. But I can't puzzle out the following: Q. When you float in fresh water, the buoyant force that acts on you is equal to your weight. When you float higher in the high-density water of Salt Lake, the B.F. that acts on you is actually: 1) greater than your...
  17. M

    Physics fluids - buoyant principle help

    A problem states that a man with a mass of 55 kg floats with 90% of his body under water. I jus need to find the weight of his body when submerged in water...i'm was thinking 90% of 55 kg, but i wasn't sure if that was correct...
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