Buoyant force Definition and 13 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. A

    I What happens to the buoyant force at the surface of the water?

    What happens to the buoyant force at the surface of the water for an object? The buoyant force should be greater than the weight of the object if the object were to float up but once the object floats to the surface, there is no more acceleration upwards which means the buoyant force = weight of...
  2. L

    Radius of hot-air balloon

    The result is supposed to be 12,2 m but every time I get 8,016 m... I used for example this formula >r=m/[(density of air-density of hot air)*(4/3)*pi] For density I used > rho=(p*M)/(R*T) Am I forgetting something? Thanks in advance.
  3. Y

    B Terminal Velocity Equation in vertical cylinder with some fluid

    I just have a question that could you guys make an equation that expresses the terminal velocity based on followed condition? - When diameter increase, velocity decrease - velocity should change depending on both cylinder and sphere's diameter - We know every variable - The sphere is in...
  4. E

    Buoyant Force and Archimedes' Principle

    Hello there, I have a quite different approach on answering the part c and d of this problem. Can you guys help me to see if my method is acceptable or logical? (I got the same answers as the back of the textbook, however my methods differ from the solutions manual). Here is my answer/attempt at...
  5. K

    Buoyant force and a water column

    Recently I viewed the Searaser video at After watching it, I feel the numbers given in the video could be wrong but not sure. So I'd like to know how I can calculate the amount of water pushed by the Searaser device at some height in one wave shot e.g. at 100 ft, 200 ft and 300 ft above sea...
  6. N

    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!
  7. Metalbob

    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...
  8. 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.
  9. J

    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...
  10. B

    Archimede's Principle and Work due to buoyant force

    Homework Statement A flotation device is in the shape of a right cylinder, with a height of 0.323 m and a face area of 4.81 m2 on top and bottom, and its density is 0.460 times that of fresh water. It is initially held fully submerged in fresh water, with its top face at the water surface. Then...
  11. S

    Error source of SHM experiment

    Homework Statement Investigating the effect of mass on the period of oscillation. This experiment is about SHM of a floating cylinder, and the theory is explained in this website: http://physics.stackexchange.com/questions/64154/shm-of-floating-objects Also, I'm attaching a diagram of my...
  12. AdityaDev

    Buoyant force doubt

    If I have an object of mass m tied to the lower surface of a vessel having a liquid and the vessel accelerates upwards... From FBD of object, Buoyant force acts upwards, mg down, pseudo force downwards (frame of reference is vessel) T down. Here's the doubt. Why is F(buoyant force) = Vp(g+a)...
  13. H

    Buoyant Force, How's my logic?

    Homework Statement A recreational (open) hot air balloon (i.e., Pinside is approximately Poutside) has a volume of 2107 m3 when fully inflated. The total weight of the balloon, basket, ballast and pilot is 1832.6 N (412 lbs). By how much must the density of the air in the balloon be smaller...