# Will buoyant force change if the container is accelerated up or down?

If we have a fluid which is in a container that is accelerated, say, upward by $$a$$. Then, what will be the buoyant force on an object with volume $$V$$, Density of liquid, $$\rho$$?

I believe it will be $$V \rho(g+a).$$ Am I right, or have I confused it with pseudo force? Can you please give a suitable explanation?

• This might help you-physics.stackexchange.com/questions/80268/… Commented Jan 12, 2014 at 12:17
• Commented Jan 12, 2014 at 12:40
• I went through the 2 links above, but I would appreciate an exact answer rather than a useful analogy...so if you could please answer this @JohnRennie, I would be grateful Commented Jan 20, 2014 at 13:02
• In addition to the answers below, I'll link this video which shows an experimental treatment of the matter in a horizontal fashion: m.youtube.com/watch?v=y8mzDvpKzfY Commented Jan 25, 2015 at 8:03
• Related demonstration: Buoyancy Paradox - University of Maryland Commented May 19, 2020 at 9:52

Archimedes' principle tells us that the upthrust on a body immersed in a fluid is equal to the weight of the fluid displaced, where the weight is the force given by $F = ma$ i.e. the mass of fluid displaced, $m$, multiplied by the acceleration, $a$, experienced by the fluid.

In this context there is no difference between gravitational acceleration and inertial acceleration - this is one example of Einstein's equivalence principle - so:

$$a = a_{gravity} + a_{inertial}$$

And the upthrust is therefore:

$$F = m (a_{gravity} + a_{inertial}) = V \rho (a_{gravity} + a_{inertial})$$

as you said in your question.

• Thanks! But now, if we take the time frame in consideration, archimedes was wrong in saying that wasn't he? Since at his time, Equivalence Principle was not given... Commented Jan 20, 2014 at 16:33
• @Rohinb97, take that! Commented Jan 20, 2014 at 16:34
• @John Rennie will this thing be true in an horizontally accelerated container also,?? I don't think it will be ...right??...or am I missing something....or putting it in another way..does force of buoyancy also depend on the horizontal acceleration of the container ??...as after reading this ..I now understand that it depends on vertical acceleration ...but I am not so sure about horizontal acceleration..? Commented Feb 5, 2016 at 3:56
• @Freelancer: this would apply to horizontal acceleration as well. See for example Why does a helium filled ballon move forward in a car when the car is accelerating?. Commented Feb 5, 2016 at 6:03

Buoyant force is actually force acting on immersed part of body due to all fluids surrounding the body and this arises due to difference in maximum and minimum pressures acting on body due to fluids around it.So when a container is accelerating upward then maximum pressure is at it,s lower surface and minimum pressure is at its top surface. I am attaching a solution .If I am wrong then give suggestions

• Sorry as image is double loaded Commented Dec 11, 2018 at 14:35
• Please typeset properly your answer. See the discussion here: physics.meta.stackexchange.com/q/10563/36194 Commented Dec 11, 2018 at 14:54

Consider a small amount of water at the surface of mass m , then when m is accelerating upward with acceleration a then from Newton second law upward resultant force acting on m is :

buoyant force - m g = ma So buoyant force = m(a+g)