pressure at the bottom of container

The four open containers shown below are all filled to the same height with water. Which container has the lowest pressure on the base of the container exerted by the water?

Container A Container B Container C Container D The pressures at the bottom of the four containers are the same

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16 solutions

Chew-Seong Cheong
Dec 10, 2017

Consider a point under water which is at a depth h h from the water surface. Consider the small horizontal surface δ A \delta A at that point. The external force in addition to that due to the atmospheric pressure on δ A \delta A is the weight of the column of water above δ A \delta A that is:

δ F = δ m g where δ m is the mass of the column of water. = ρ h δ A g where ρ is the density of water. δ F δ A = ρ g h As δ A d A P = d F d A = ρ g h where ρ pressure at the point h below the water pressure. \begin{aligned} \delta F & = \delta mg & \small \color{#3D99F6} \text{where }\delta m \text{ is the mass of the column of water.} \\ & = \rho h \delta A g & \small \color{#3D99F6} \text{where }\rho \text{ is the density of water.} \\\frac {\delta F}{\delta A} & = \rho gh & \small \color{#3D99F6} \text{As } \delta A \to dA \\ \implies P & = \dfrac {dF}{dA} = \rho gh & \small \color{#3D99F6} \text{where }\rho \text{ pressure at the point }h \text{ below the water pressure.} \end{aligned}

Note that since ρ \rho and g g are constants, P P is only dependent of h h , the height. Since water is filled to the same height in the four containers, the pressure at the bottom of the four containers are the same.

Note also that when δ A d A \delta A \to dA , d A dA can be facing any direction, the pressure is the same. Therefore, pressure at a point in a liquid is equal in all directions.

But the boundaries of the container D are subjected to height less than h due to the shape of D. Shouldn't it have less pressure?

Mohamed Hessien - 3 years, 6 months ago

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We are talking about the water pressure in the container not on the floor the containers lie on. Liquid pressure is always given by ρ g h \rho gh .

Chew-Seong Cheong - 3 years, 6 months ago

If you consider a small horizontal surface that is below one edge of the tank in D then h would be less than the height of the tank. No? So your equation doesn't hold for any area on the bottom that does not have a direct column above it all the way to the top

Tristan Minifie - 3 years, 6 months ago

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But when it is infinitesimally, it is same in all directions. That is d A 0 dA \to 0 .

Chew-Seong Cheong - 3 years, 6 months ago

So do you mean a frustrum completely filled having height "h" experiences the same pressure as ac cylinder filled till the same height h. Even if their base radius are same

Or If their top radius are same

I consider your ideology wrong

Janesh G - 3 years, 6 months ago

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The pressure refer to liquid pressure in the container not on the floor. Liquid is given by ρ g h \rho gh .

Chew-Seong Cheong - 3 years, 6 months ago

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Chew its mention3d pressure at the base of container Not the loquid pressure So more pressure

Janesh G - 3 years, 6 months ago

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@Janesh G Not base but bottom, which is internal. Just like the bottom of a swimming pool.

Chew-Seong Cheong - 3 years, 6 months ago

What if you visualized the base as a stopper into a vertical tube with cross-sectional area equal to the bases A, B, C and D. The greatest pressure out of the tube once the stopper is removed give the column of water is high enough would be in Case C. Inversely, the lowest pressure of water would be D, going from smaller to larger, correct?

Am I missing something?

Michael Fitzgerald - 3 years, 6 months ago

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We are talking about the pressure at a point at the bottom of the container (inside the container, just as the bottom of a swimming pool) not the floor under the base.

Chew-Seong Cheong - 3 years, 5 months ago

I totally agree with your solution but I have one question . We know that pressure is force per unit area so we can say that the container having largest bottom surface area will have lowest bottom pressure ?? And if we apply this here then we are getting some other answer .

Nashita Rahman - 3 years, 5 months ago

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We are talking about the pressure at a point not an area at the bottom of the container (inside the container, just like the bottom of a swimming pool). Every point at the bottom in water has the same pressure.

Chew-Seong Cheong - 3 years, 5 months ago

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If this is the case, the wording of the question should properly reflect this. As it is now, it is unclear if the question is asking about a single point, or about the area at the bottom of the container. Ambiguity creates the confusion seen in these comments.

Brandt Aulken - 3 years, 5 months ago

Inside the bottle on the botom. You missed the "inside" word

Zeljko Banovic - 3 years, 5 months ago

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There fore the right answer is b . Better concentration next time...

Zeljko Banovic - 3 years, 5 months ago

Here is my question: suppose that you removed a column B shaped section of water out of shape C. The remaining water (along the edges) would fall to the bottom of shape C. Therefore, that water, even now, must be exerting a downward force. Why wouldn't that downward force (of the extra water that C has over B) add to the force of the B-shaped column of water to make more total pressure on the bottom of Shape C? That's the thing that has me confused about this one.

Chris Foster - 3 years, 5 months ago

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The problem refers to the water pressure at the bottom, which only depends on the height of the water column. It does not refer to the the pressure on the floor because of the weight of the containers which depends on the volume of water and the area of the base.

Pranshu Gaba - 3 years, 5 months ago

What confused me is i was thinking of pressure as weight per unit of area...since the weight of the water in D was less than in A and the surface area was larger then the others then the pressure is lowest inD

Greg Grapsas - 3 years, 5 months ago

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It would have been true, if it is solid and not liquid.

Chew-Seong Cheong - 3 years, 5 months ago

I feel I must disagree here. As far as I understand it, pgh is only applicable under the assumption that every column of water is vertical. This assumption doesn’t hold here. Imagine the following: You begin with setup C. Now You freeze the entire thing, except You leave the bottom delta (in height) of water liquid. The pressure on that bottom bit should be the same before and after freezing. But this change makes it clear that that pressure depends on the mass of water above it. If we cut out the non-vertical bits (leaving only a cylinder), the object on top of the water is clearly now less massive and therefore exerting less force/area than before.

Of course, a and b, for example, do obviously have the same pressure.

David Lalo Rudman - 3 years, 5 months ago

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Another argument: If You take the column-of-water idea bad actually go through every infinitesimal square on the bottom, You will only have accounted for the pressure increase cause by water directly above each piece of area You calculated. That is, all the pressure of the water on the sides will now be unaccounted for. This is problematic because the water on the sides does not exert a purely horizontal force—this is clear because that water on the sides must be supported by something, and, it can’t be held up by the sides of the container because the normal at those points is not perpendicular to the floor.

Another illustrative example: imagine it were sand rather than water (there same physics should apply in the limit as Your particles of sand get infinitely small). If You have a cylindrical pile of sand on top of the base of c, and then You place a heavy marble on the side part (assuming it somehow does not knock down the cylinder), it clearly has a force component that points towards some point on the bottom of the base. There will therefore obviously be increased force on the cylinder in that direction, suggesting that any pressure it places on the bottom of the bowl (and therefore the infinitesimal layer of sand above it) will increase.

David Lalo Rudman - 3 years, 5 months ago

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"because the water on the sides does not exert a purely horizontal force—this is clear because that water on the sides must be supported by something" This is incorrect. By Newton's third law, there is an equal pressure from the container wall. Consider a point (an infinitesimal sphere) in the liquid there is an equal and opposite pressure in all directions. That is why the infinitesimal sphere does not move around at equilibrium. That is why pressure at in all direction equally. If you puncture the vertical wall of the container water shoots out horizontally. I used the column of water just for easy of calculation. Once I get the value the pressure in all other directions is the same.

Chew-Seong Cheong - 3 years, 5 months ago

isn't it depends on the area of the base of the container?

Mutiara Nurlena - 3 years, 5 months ago

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It is not the pressure on the floor but the water pressure on the bottom of the container. Liquid pressure only depends on ρ g h \rho g h .

Chew-Seong Cheong - 3 years, 5 months ago

The total pressure at any depth of an open container is equal to the atmospheric pressure plus the unit weight of liquid multiplied by the height from the free surface. Since the height is the same and the kind of liquid is the same, the pressures at the bottom of the containers are also the same.

Note: The difference in pressure between any two points in a homogeneous fluid (constant unit weight) at rest varies directly as the difference in depth or elevation of the two points.

But the containers have different amounts of liquid. Less weight less pressure and vice versa

Jody Mayfield - 3 years, 6 months ago

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Here pressure is depend on only height. I gave a solution here. You can check it.

Md Mehedi Hasan - 3 years, 6 months ago

The question is somewhat ambiguously referring to water pressure, not the pressure the container would exert on the ground.

Michael Keefe - 3 years, 6 months ago

Ofcourse you are right Marvin. But can I just try and outsmart you by stating that a real measurement would slightly disagree with your mathematically correct answer? Since any measuring device would ofcourse take up a small amount of space at the bottom of each glas, therefore pushing the water outward, and since each glas has different diameters, the amount that the water would rise due to the pushing of this device would differ a little bit. So I believe in a reallife situation cup A would measure a little less pressure than the others because it simply has more room available for the water to spread out instead of up. ;) Highest regards, David Bartenstein

David Bartenstein - 3 years, 6 months ago

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The volume above being less than below cf with the other options?

John Barry Robinson - 3 years, 6 months ago

This answer makes sense. The "total pressure" is the same assuming the volume of water above each of the surfaces is the same

Michael Fitzgerald - 3 years, 5 months ago

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So in the question they only said “to the same line” but doesn’t say they all have the same volume of water.

Aaron Slagman - 3 years, 5 months ago

What do you mean by 'Unit weight of the Liquid?'

Jordan Castelino - 3 years, 5 months ago

Unless further pressure is forced onto the container.

Chuck Ardenstrike - 3 years, 5 months ago

Since the pressure of the water is vertical, and the four containers are filled to the same height, then the pressure at the bottom of the four containers are the same.

Good answer :)

Felipe Correa Gomes - 3 years, 6 months ago

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No, pressure works in all directions. Punctual a side of the tank filled with water and water will shoot out horizontally. See my solution below.

Chew-Seong Cheong - 3 years, 6 months ago

Very good solution Pablo, here have an upvote

Albert Zacho - 3 years, 6 months ago

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No, pressure works in all directions. Punctual a side of the tank filled with water and water will shoot out horizontally. See my solution below.

Chew-Seong Cheong - 3 years, 6 months ago

A very good solution, Pablo! - Shawn

Shawn Dobberfuhl - 3 years, 6 months ago

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No, pressure works in all directions. Punctual a side of the tank filled with water and water will shoot out horizontally. See my solution below.

Chew-Seong Cheong - 3 years, 6 months ago

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Thanks for letting me understand this. I didn't really get it until now!

Shawn Dobberfuhl - 3 years, 6 months ago

Pressure works in all directions. Punctual a side of the tank filled with water and water will shoot out horizontally. See my solution below.

Chew-Seong Cheong - 3 years, 6 months ago

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But don't they all have different masses of water in them? Therefore more weight acting on the bottom?

Guy Denton - 3 years, 6 months ago

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Pressure is the same in all direction. Because water is a liquid. See my solution below. It only depends on h h .

Chew-Seong Cheong - 3 years, 6 months ago

Oh, I thought that the pressure of water, just like air only goes down instead of horizontally. That is why since there is more water in some containers, they are still all the same; because the pressure of the water is vertical.

Pablo Romero Alcaine - 3 years, 6 months ago

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No, we can do an experiment to show this. If you fill up a balloon with water. Punctual it with holes all over. Every holes will shoot out water including those on top. Check my solution below.

Chew-Seong Cheong - 3 years, 6 months ago

Even air is also the same pressure in all directions. If down air will keep flowing down.

Chew-Seong Cheong - 3 years, 6 months ago

Since the pressure of the water is vertical, and the four containers are filled to the same height, then the pressure at the bottom of the four containers are the same.

Felipe Correa Gomes - 3 years, 6 months ago

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No, pressure works in all directions. Punctual a side of the tank filled with water and water will shoot out horizontally. See my solution below.

Chew-Seong Cheong - 3 years, 6 months ago

Does the volume of water change the pressure?

Aaron Slagman - 3 years, 5 months ago

Um.... the volume is not the same tho ... and containers with slanted sides will have different pressures on base ... I don't get it ...

Marina Longnickel - 3 years, 6 months ago

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See my solution below.

Chew-Seong Cheong - 3 years, 6 months ago

Okay, here's the next question - what if each container were slanted at say at 45 degree angle?

Jim Witte - 3 years, 6 months ago

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Its just the height of the water column above the bottom point of interest (+ atmospheric) - its that simple!

Brian M - 3 years, 5 months ago
Swafim Li
Dec 13, 2017

there is a concept easy to be misunderstood.

what does the "pressure" mean here?

if it means "density of pressure"(which is written with the letter P), the answer should be the last one.

but if it means "a kind of force"(written with the letter F), the answer should be B and C, for their area of bottom is the smallest. the equation F=PS tells us when P is same, force F decreases with the decreasing of area S.

I assumed the answer was dependent upon "pressure density," figure D. If the question were "Which container has the greatest pressure" then all would be the same.

Marc Kamens - 3 years, 5 months ago

I thought they meant total pressure on bottom area, and my answer was B.

Nemanja Stojanović - 3 years, 5 months ago
Vishal Dbhat
Dec 11, 2017

This effect us known as the hydrostatic paradox by virtue of which irrespective of the area of the cross section of the container in which the liquid is enclosed, the pressure exerted remains the same provided the volume of the liquid in each container remains the same

The volume is not the same tho...

Marina Longnickel - 3 years, 6 months ago
Md Mehedi Hasan
Dec 10, 2017

The pressure in bottom of container is depend on height.

It maintains this formula,

p r e s s u r e h when g and density of water is constant. \large pressure\propto h\quad \boxed{\text{when g and density of water is constant.}}

From this formula we can see, pressure is not depend on the size of container besides height.

Because of being same height, the answer must be The pressures at the bottom of the four containers are the same \boxed{\color{#20A900}\text{The pressures at the bottom of the four containers are the same}}

Arianna Colella
Dec 17, 2017

Pressure is defined as the force applied perpendicular to the surface of an object per unit area over which that force is distributed. In other words, we can think of pressure in this situation as the force applied by a column of water that extends over each square centimetre on the base of the container. If we think of it this way, it becomes clear that the pressure exerted over the base of each container is equal to the pressure being exerted by each "column" of water, and has nothing to do with the surface area of the base. Since the amount of pressure exerted by each column depends only on its height, and each container is filled to the same height with the same liquid, we can say that the pressures at the bottom of the four containers are the same.

Gayatri Gupta
Dec 16, 2017

Since pressure depends on three quantities height,density of liquid acceleration due to gravity in the formula P=hdg. H is constant in all four containers so P at the bottom will be same in all the cases.

Matteo D'Andria
Dec 15, 2017

Hydrostatic pressure depends only on the depth; thus, theoretically speaking, it should be it for gases too, due to the nature of bonds between molecules. In our study of thermodynamics though we neglect the difference of pressure in an ideal gas between two different heights. This hypotesis makes sense considering the very low density of gases.

Sourav Saha
Dec 15, 2017

Simply Pascal law says that pressure acting at all molecules in an horizontal plane is equal and ass all are in a same plane so pressure experienced by all of them in bottom will be same

P = u.g.H, u - density, g - gravity, H - liquid height All the liquids are the same, so the density are equal in each one of them. Gravity its equal too The liquid height it's the same.

So, the pressure in the bottom of them are equal.

Gaurav Singh
Dec 13, 2017

Pressure is given by pgh. Since the height and density of the liquid column is same the pressure will be same in all the three cases.

Aryan Misra
Dec 13, 2017

Its called hydrostatic paradox in pascal law Where under ideal condition pressure in every of different shape of vessel is same as it is shown above.

We know, P=h rho g i.e., Pressure is the multiplication of height and density of the liquid, times acceleration due to gravity on that place. So, pressure does not depend on the area of the bottom, results the answer.

Willie le Grange
Dec 11, 2017

In Engineering, the pressure for a fluid at a specific height is calculated as ρ \rho gh. This means that the only variables that do play a role in the pressure at the bottom of the tank is density, gravitational acceleration and height . The shape will therefore not influence the pressure at the bottom of the tank.

This has always puzzled me! So say the area at the base were 1m², and the upper 990mm of the container was only 10mm diameter, what would the pressure be per m²? Surely it could not be 1000 kg (yes I know that's mass not force!) per m², if the mass of water is only a few kg?

Ian Chappel - 3 years, 6 months ago
Michael Coffee
Dec 11, 2017

The equation for pressure is the density of the liquid * gravity * height. They all have the same liquid and gravity and height, therefore, they have the same pressure

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