iTeachChem•9mo ago

Fluids + NLM

wont net force on this be 0? psudeo force + mg = Boyouncy Force
so shouldnt ans be D? since it will inf time

iTeachChem Helper•4/20/25, 3:19 AM

@Gyro Gearloose

iTeachChem Helper•4/20/25, 3:19 AM

Note for OP

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GGamertug wont net force on this be 0? psudeo force + mg = Boyouncy Force so shouldnt ans...

SirLancelotDuLac•4/20/25, 3:20 AM

Initial position mein mg=Buoyancy, but when container is accelerated and block is released from the original position, there is an imbalance of pseudo force.

SirLancelotDuLac•4/20/25, 3:20 AM

This imbalance pushes the block upwards.

SSirLancelotDuLac Initial position mein mg=Buoyancy, but when container is accelerated and block i...

GamertugOP•4/20/25, 3:22 AM

it said it accelrated first , then at some instant block is released

GGamertug it said it accelrated first , then at some instant block is released

SirLancelotDuLac•4/20/25, 3:23 AM

Yes. The block is kept at its initial position (by some unspecified force) and then released, whence the imbalance pushes the block.

SSirLancelotDuLac Yes. The block is kept at its initial position (by some unspecified force) and t...

GamertugOP•4/20/25, 3:23 AM

what imbalance

GGamertug what imbalance

SirLancelotDuLac•4/20/25, 3:24 AM

The one due to pseudo-force.

SirLancelotDuLac•4/20/25, 3:24 AM

Or rather the increased buoyant force due to increase in g_effective.

SSirLancelotDuLac The one due to pseudo-force.

GamertugOP•4/20/25, 3:24 AM

bro assume mg = F/2 , ma is also F/2 , Boyuoncyy force = F , all cancel out

SSirLancelotDuLac Or rather the increased buoyant force due to increase in g_effective.

GamertugOP•4/20/25, 3:24 AM

when taking psudeo force why would g effective change?

GGamertug when taking psudeo force why would g effective change?

SirLancelotDuLac•4/20/25, 3:25 AM

*Rather than taking pseudo-force if you want to do by g_effective in that case

SSirLancelotDuLac *Rather than taking pseudo-force if you want to do by g_effective in that case

GamertugOP•4/20/25, 3:25 AM

ah ic

GamertugOP•4/20/25, 3:25 AM

still

GamertugOP•4/20/25, 3:25 AM

it will balance

GamertugOP•4/20/25, 3:25 AM

idk why it will move

SirLancelotDuLac•4/20/25, 3:27 AM

WhatsApp_Image_2025-04-20_at_08.57.39_8fce5c75.jpg

SirLancelotDuLac•4/20/25, 3:28 AM

(Via the g_eff approach)

SirLancelotDuLac•4/20/25, 3:28 AM

Initially it is kept at 10m depth by unspecified forces.

SSirLancelotDuLac Click to see attachment

GamertugOP•4/20/25, 4:14 AM

but what about the case where we take psuedo force , why is force 0 there , we get inf time , what is wrong in that approach

g-sus•4/20/25, 9:04 AM

i think the q is saying that the block accelerates with 10, this isnt the same 10 that cancels g, theres 20 units of acceleration upwards

Gg-sus i think the q is saying that the block accelerates with 10, this isnt the same 1...

GamertugOP•4/20/25, 9:40 AM

no it def says the vessel is acc , not the block

g-sus•4/20/25, 10:05 AM

fair enough, i didnt note that

GGamertug but what about the case where we take psuedo force , why is force 0 there , we g...

SirLancelotDuLac•4/20/25, 12:33 PM

Vessel is accelerating upwards right? Now the pseudo-force on the block is downward, but there is also pseudo-force (downward) on the liquid, so the liquid exerts more buoyant force on the block. (Because now pressure gradient is higher with depth). So, the forces do not cancel out.

SSirLancelotDuLac Vessel is accelerating upwards right? Now the pseudo-force on the block is downw...

GamertugOP•4/20/25, 2:40 PM

OOOH

SSirLancelotDuLac Vessel is accelerating upwards right? Now the pseudo-force on the block is downw...

GamertugOP•4/21/25, 3:05 AM

wait , psudeo force is downward on liquid , so it will become more dense , and because of more dense bouoynt force will increase thats why forces dont cancel , but how much denser will it become? cant say anything about that

GGamertug wait , psudeo force is downward on liquid , so it will become more dense , and b...

SirLancelotDuLac•4/21/25, 3:18 AM

A rather correct way than saying "more dense" (since fluid is incompressible) would be the pressure gradient increases. As for by how much does it increase, consider a thin layer of width dy, then pressure dP on it would be rho.A.dy.(g+a)/A, so dP/dy=rho.(g+a) (where a is acceleration of the vessel)

SSirLancelotDuLac A rather correct way than saying "more dense" (since fluid is incompressible) wo...

GamertugOP•4/21/25, 3:21 AM

ok and how will that pressure gradient affect force?

GGamertug ok and how will that pressure gradient affect force?

SirLancelotDuLac•4/21/25, 3:22 AM

Bouyancy is due to pressure difference, so it does affect buoyancy which affect the force on the object.

SSirLancelotDuLac Bouyancy is due to pressure difference, so it does affect buoyancy which affect ...

GamertugOP•4/21/25, 3:22 AM

isnt it Vrhog

GGamertug isnt it Vrhog

SirLancelotDuLac•4/21/25, 3:24 AM

Consider a cylinder. The pressure upwards on the bottom face is P(d+h)*A while that at the top is P(d).A (where P(x) is pressure at depth x, d is depth of top face and h, A are height and area of cylinder). So buoyant force is [P(d+h)-P(h)]A.

SirLancelotDuLac•4/21/25, 3:24 AM

If the pressure is rho.g.h, then it becomes rho.g.h.A=rho.V.g

SSirLancelotDuLac A rather correct way than saying "more dense" (since fluid is incompressible) wo...

SirLancelotDuLac•4/21/25, 3:25 AM

Since here the pressure is 2rho.g.h (which you can see from here), the bouyancy is 2rho.V.g

SSirLancelotDuLac Consider a cylinder. The pressure upwards on the bottom face is P(d+h)*A while t...

GamertugOP•4/21/25, 3:26 AM

well , due to psudeo force pressure will increase ye , but the diffrence in pressure will still remain same no?

GGamertug well , due to psudeo force pressure will increase ye , but the diffrence in pres...

SirLancelotDuLac•4/21/25, 3:26 AM

No, the rate at which pressure increases increases. So pressure difference will indeed increase.

SSirLancelotDuLac No, the rate at which pressure increases increases. So pressure difference will ...

GamertugOP•4/21/25, 3:28 AM

i dont quite understand this

GGamertug bro assume mg = F/2 , ma is also F/2 , Boyuoncyy force = F , all cancel out

SirLancelotDuLac•4/21/25, 3:31 AM

Well buoyancy is due to pressure difference at two points right? Now the pseudo-force increases the pressure exerted by each thin layer of fluid; so pressure increases more quickly as compared to the normal condition here. So pressure difference between 2 points with same vertical distance is more in this case as compared to the normal condition, hence buoyancy (which is due to pressure difference) increases in this case. This increased buoyancy is the answer to what you did not take into account here.

SSirLancelotDuLac Well buoyancy is due to pressure difference at two points right? Now the pseudo-...

GamertugOP•4/21/25, 3:33 AM

Well my counter argument is that lets take a point A it has lets say 0 pressure due to liquid i took base here , and pusedo force
now point B at height H , it has pressure Rho G and puseodo force
the psudeo force is same in both , so the pressure diffrence remains constant

GGamertug Well my counter argument is that lets take a point A it has lets say 0 pressure ...

SirLancelotDuLac•4/21/25, 3:35 AM

Okay, Imagine a cylinder of height h from B to A. If you say the pressure at bottom is rho.g.h times higher than at B, the fluid experiences a force of rho.g.h.A in upwards direction (from ground frame, that is). This is balanced by the force of gravity so the cylinder should not have any motion in ground frame.

SirLancelotDuLac•4/21/25, 3:35 AM

The correct thing would be the pressure difference is rho.h.(g+a). (Which is in line with the g_effective wali approach)

GamertugOP•4/21/25, 3:41 AM

uid experiences a force of rho.g.h.A in upwards direction (from ground frame, that is). This is balanced by the force of gravity so the cylinder should not have any motion in ground frame. , i dont understand this part

GamertugOP•4/21/25, 3:43 AM

i still dont understand where there would be a pressure diffrence at all

GamertugOP•4/21/25, 3:43 AM

psudeo force will remain constant throught the liquid

SirLancelotDuLac•4/21/25, 3:47 AM

WhatsApp_Image_2025-04-21_at_09.17.33_d5ffbeba.jpg

GGamertug uid experiences a force of rho.g.h.A in upwards direction (from ground frame, th...

SirLancelotDuLac•4/21/25, 3:49 AM

This is from the ground frame. (The image) If we take inertial frame, then the explanation would be similar. (Pseudo-force in downward direction+weight of liquid) is balanced by extra pressure exerted on the fluid by the bottom layer.

SSirLancelotDuLac Click to see attachment

GamertugOP•4/21/25, 3:51 AM

where u get this from?

GGamertug where u get this from?

SirLancelotDuLac•4/21/25, 3:52 AM

Force by the bottom layer is (P+dP)A while that by the top layer is PA

SirLancelotDuLac•4/21/25, 3:53 AM

So just force balancing, dP.A-mg=ma and since m=rho.A.dy for a thin layer of dy thickness, dP.A=rho.A.dy(g+a)

SSirLancelotDuLac So just force balancing, dP.A-mg=ma and since m=rho.A.dy for a thin layer of dy ...

GamertugOP•4/21/25, 3:54 AM

ah ic

SSirLancelotDuLac Force by the bottom layer is (P+dP)A while that by the top layer is PA

GamertugOP•4/21/25, 3:55 AM

ye mathematically i can see the result , but in the physical sense i still dont get it , like the pusedo force is Constant it will just get canceled out when taking change