Confused about the pressure formula

[jayadds]

Hi,

I've come across two different formulae for pressure:

p = p0 + rho*g*h (p0 = initial pressure)

OR

p = rho*g*d

My question is, why does one of the formulae includes the initial pressure (initial pressure is often the atmospheric pressure = 1.013*10^5 Pa) in the formula whereas the other one doesn't? Do they actual calculate the same thing?

Many thanks.

 

[Doc Al]

The second version just gives the pressure due to the fluid (density rho). If you want the full pressure, you need to add the pressure above the fluid, p0.

 

[jayadds]

The second version just gives the pressure due to the fluid (density rho). If you want the full pressure, you need to add the pressure above the fluid, p0.

Having said that, then for the following question should I use the formula with or without the atmospheric pressure?

A research submarine has a 20 cm diameter window. It can withstand forces up to 1.0 x 10^6 N. What is the submarine's maxiumum safe depth in water?

 

[Doc Al]

Depends. Do they mean the inward force from the water or the net inward force? (Presumably, the pressure inside the submarine is atmospheric.) Lacking additional information, I would use the one without the atmospheric pressure.

 

[sophiecentaur]

Personally, I'd use the 'pessimistic' version. But I may be just a wimp.