# Need help from physics gurus (Spec grav & PSI)

Discussion in 'Middle School / Junior High' started by trina, Nov 28, 2007.

1. ### trinaCompanion

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Nov 28, 2007

Hi there,

There's a problem in our 8th grade science book that I don't understand how to solve. Of course the teacher's edition gives me the answer, but with no explanation. Just the answer. Since this subject matter is not my strong suit, I need some help.

Question: The specific gravity of liquid hydrogen is 0.07. If the pressure at the bottom of a 50 ft high tank of water is 22 psi, what would be the pressure at the bottom of the same tank if it were filled with liquid hydrogen instead?

Book's answer: 1.54 psi (0.07 x 22 psi)

The only formulas explained in the text just before this question are

specific gravity= density of water/density of water
and
pressure= force/area

I don't think either of these formulas apply to this; however, I may be wrong. And if one does apply, please explain how.

Thanks for any help! I can't stand telling kids "that's the answer because the book says it is!"

3. ### Carmen13Groupie

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Nov 30, 2007

I replied to you yesterday! What happened to my post?

It was a long one and I didn't save it!

4. ### Carmen13Groupie

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Nov 30, 2007

I tried to manipulate the formulas you gave, to obtain the answer.

Formulas:

density= mass / volume d=m/V

pressure=Force/area= (mass*gravity force)/area p=F/A=(mg)/A

specific gravity=substance density / water density

Let's use these abbreviations:

d_H = hidrogen density

d_W = water density

m_H = hidrogen mass

m_W = water mass

_______________

We know that

p_H= F/A = (m_W*g)/A

On the other hand

d_W=(m_W*g)/A <=> A=(m_W*g)/22 (since d_W=22)

Substituting this on the first equation we get

p_h= (m_H*g)/(m_W*g/22) = m_H/m_W * 22

= (m_H*V)/(m_W*V)*22= d_H/d_W * 22 = 0,07 * 22

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