Need help with this physics problem

Discussion in 'High School' started by trina, Nov 29, 2007.

  1. trina

    trina Companion

    Jan 16, 2007
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    Nov 29, 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
    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. 3Sons

    3Sons Enthusiast

    Aug 30, 2007
    Likes Received:

    Nov 29, 2007

    Maybe you're overthinking. I'm not by any means a physics expert, but as I recall water has a specific gravity of 1, and everything else is based off that. I think you mean specific gravity is the density of the fluid over the density of water.

    So, if you have 22 psi/1 = x/.07. Solve for X.
  4. Carmen13

    Carmen13 Groupie

    Jul 6, 2005
    Likes Received:

    Nov 29, 2007

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


    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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