Pressure Required for Fireman to Send Water 12m High | Physics Problem

[Raffaele]

<Moderator's note: Moved from a technical forum and thus no template.>

My daughter, who attends the italian equivalent of US K9, asked me some advice about a physics hydrostatic problem.

> Which pressure (in atmospheres) a fireman must use to send water at 12
> meters height.

I tried to use the definition 76 cm of mercury is 1 atm and I made the proportion between water and mercury densities

Namely

if with a density of 13.5
atmosferic pressure raises Hg to 0.76 m then the pressure of 1 atm raises water to 13.5\times 0.76 m that is 10.26m.

Thus to arrive to 12m the pressure required is 12/10.26=1.17 atm

But the book says 2.2atm

Can you, please, explain what I did wrong?

Thanks in advance

 

[DoItForYourself]

<Moderator's note: Moved from a technical forum and thus no template.>

My daughter, who attends the italian equivalent of US K9, asked me some advice about a physics hydrostatic problem.

> Which pressure (in atmospheres) a fireman must use to send water at 12
> meters height.

I tried to use the definition 76 cm of mercury is 1 atm and I made the proportion between water and mercury densities

Namely

if with a density of 13.5
atmosferic pressure raises Hg to 0.76 m then the pressure of 1 atm raises water to 13.5\times 0.76 m that is 10.26m.

Thus to arrive to 12m the pressure required is 12/10.26=1.17 atm

But the book says 2.2atm

Can you, please, explain what I did wrong?

Thanks in advance

Hi,

the answer is 2,2 because you must add the atmospheric pressure (1 atm) to the needed ΔP that you have already calculated.

Also, have in mind that water needs approximately 1 atm ΔP in order to be elevated at 10 m height ( this is a rule of thumb).

 

[CWatters]

The equation to use is

P=hdg

Where h is height
d is the density
g is acceleration due to gravity.

Remembering to add for atmospheric pressure as Raffaele mentioned.