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Homework Help: Ball of gas experiences rapid increase in pressure...

  1. May 24, 2016 #1
    1. The problem statement, all variables and given/known data
    A self-gravitating ball of gas suddenly experiences a rapid increase in its pressure to a new value P'. Taking average density of the star to be ρ and radius R, and ignoring gravity altogether, estimate the time scale of expansion of the star.

    2. Relevant equations
    Force= Pressure/Surface area= mass x Acceleration

    3. The attempt at a solution
    Considering an outer shell of thickness δr and radius r:
    Outward force on shell = (P(r) - P(r + δr)) 4πr2
    My question is really why is the P(r + δr) the minus of it?

    I'm comfortable doing the rest of the question by just using equation mass of shell x acceleration and equating them and using dimensional analysis on that .
    But I'm unsure why it's a minus there, i'm fairly certain that's right after speaking classmates but I don't understand why.

    Thanks in advance.
  2. jcsd
  3. May 24, 2016 #2


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    2017 Award

    Because it works inwards ! Make a little sketch for a thin shell at r .
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