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Force exerted on walls of metal box please help

  1. Dec 30, 2007 #1
    force exerted on walls of metal box...please help!!!

    1. The problem statement, all variables and given/known data
    A cubic metal box that has 20-cm-long edges contains air at a pressure of 1.0 atm and a temperature of 300 K. The box is sealed so that the enclosed volume remains constant, and it is heated to a temperature of 400 K. Find the force due to the internal air pressure on each wall of the box.




    2. Relevant equations
    PV=nRT




    3. The attempt at a solution
    F=[tex]\left|[/tex][tex]\Delta[/tex][tex]\vec{p}[/tex]/[tex]\Deltat[/tex][tex]\right|[/tex]
    v=[tex]\sqrt{3RT/M}[/tex]=[tex]\sqrt{3(8.314 J/molxK)(300K)/0.029kg/mol)}[/tex]=507.957 m/s
    Delta[/tex][tex]\vec{p}[/tex]/[tex]=mv^2=(9423.9987g)(507.957m/s)=4.787x10^6
    I just need delta t
     
  2. jcsd
  3. Dec 30, 2007 #2

    Dick

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    Forget about v. You don't need to count individual particles, PV=nRT will give you the pressure. Pressure is force per unit area.
     
  4. Dec 30, 2007 #3
    How do I use all the information given in the problem to find that?
     
  5. Dec 30, 2007 #4

    Dick

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    PV=nRT. How many variables in this equation change when the temperature is varied in your problem?
     
  6. Dec 30, 2007 #5
    I think the pressure would change but the volume remains constant in the box. Is the pressure given in the problem negligible or should I be able to use it?
     
  7. Dec 30, 2007 #6

    Dick

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    Only P and T change, right. But you are given the initial P is 1atm. What's the final P? You certainly shouldn't neglect the pressure change!
     
  8. Dec 30, 2007 #7
    got it...thanks!!!
     
  9. Jul 2, 2008 #8
    Re: force exerted on walls of metal box...please help!!!

    I'm having severe trouble on this one:

    I know that P1(T1)/T2 = P2

    so P2 x (length)^2 x (6 sides) = Internal Pressure Force.

    But I'm not getting the right values. Even if I just do P2 x (Length^2) i don't get the right answer.
     
  10. Jul 2, 2008 #9

    Dick

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    Re: force exerted on walls of metal box...please help!!!

    Show your whole solution. We can't really be expected to guess what you did wrong.
     
  11. Jul 2, 2008 #10
    Re: force exerted on walls of metal box...please help!!!

    101300 (400) / 300 = P2 = 135093

    135093 Pa x (.20^2) x 6 = 32422 N

    and the other way 135093 Pa (.2^2) = 5403 N

    neither worked
     
  12. Jul 2, 2008 #11

    Dick

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    Re: force exerted on walls of metal box...please help!!!

    I don't know. I got 5403N as well. Significant figures issue??
     
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