Pressure 2 m deep in a tank full of water

[SpiffyPhysics]

Homework Statement

In a tank full of water, the pressure on a surface 2 meters below the water level is
1.5 kPa. What is the pressure on a surface 6 meters below the water level?

Homework Equations

Isn't this impossible? The equation given is P=hdg. (height of water column x density x 9.8m/s/s) and the density of water is 1.

The Attempt at a Solution

So by that equation, the pressure 2 meters below would be P=2m x 1 x 9.8m/s/s = 19.6 kPa
and the pressure 6 meters below the surface would be P= 6m x 1 x 9.8m/s/s = 58.8 kPa

Am I misunderstanding something here?

Thank you in advance!

 

[mfb]

The equation given is P=hdg.

This equation gives zero pressure at a height of 0. Possible, but it is a special case that does not have to be true. There is a more general formula. Alternatively, consider pressure differences only.

 

[haruspex]

The planet is not specified. But then, neither is its atmospheric pressure, making the whole indeterminate.

 

[mfb]

I guess we can assume this happens at the surface of Earth. Atmospheric pressure does not influence the result (but we have to assume that 1.5 kPa are pressure relative to atmospheric pressure otherwise the described system cannot exist).

 

[haruspex]

I guess we can assume this happens at the surface of Earth. Atmospheric pressure does not influence the result (but we have to assume that 1.5 kPa are pressure relative to atmospheric pressure otherwise the described system cannot exist).

But Spiffy's point is that the set up described is not possible under those conditions. The excess pressure would be more like 20kPa at 2m. That's why I brought up the possibility of a different context.

 

[mfb]

Excess pressure relative to the highest point in the tank, which can be below atmospheric pressure.

 

[haruspex]

Excess pressure relative to the highest point in the tank, which can be below atmospheric pressure.

Sure, but it can't be negative. Even if the tank is under a vacuum the pressure at 2m will be far more than the question states.

 

[mfb]

Sometimes pressure is given relative to atmospheric pressure. I suggest to wait for SpiffyPhysics.

@SpiffyPhysics: Is this the full and exact problem statement?

 

[haruspex]

Sometimes pressure is given relative to atmospheric pressure. I suggest to wait for SpiffyPhysics.

@SpiffyPhysics: Is this the full and exact problem statement?

Let the absolute pressure at the surface be P. On Earth, the pressure at 2m depth of water will be about P+20kPa, yes? What value of P is going to give a resulting pressure of a mere 1.5kPa?

The only interpretation that makes sense to me is a) this is not on Earth, and b) the pressures are relative to atmospheric, whatever that is.

 

[ehild]

Let the absolute pressure at the surface be P. On Earth, the pressure at 2m depth of water will be about P+20kPa, yes? What value of P is going to give a resulting pressure of a mere 1.5kPa?

The only interpretation that makes sense to me is a) this is not on Earth,

Or the fluid is not water.

 

[TSny]

The pressures are gauge pressures and the tank is in an elevator.

 

[haruspex]

Or the fluid is not water.

It says water.

 

[haruspex]

The pressures are gauge pressures and the tank is in an elevator.

An accelerating elevator... Albert E says that might as well be a different planet.