Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Help! Conservation Of Mass

  1. Mar 20, 2005 #1
    :smile: Hi all
    I am in deperate, desperate, desperate need of some help. I have this question that I have been working on for hours and have made no progress at all.

    Here is the question.

    Write down the "conservation of mass" equation for dm(r)/dr, where m(r) is the mass inside radius "r". Assume that the pressure inside a star at radius r is given by

    P(r) = Pc/R(R-r)

    where Pc is the central pressure and R is the stars outer radius. Combine this and the equation of hydrostatic equlibrium to find an expression for m(r). Hence show that m(r) x r5/2

    PLease please help,

    any advice on how to solve this or the answer will be great.

    Thanks All

  2. jcsd
  3. Mar 20, 2005 #2
    The equation for hydrostatic equilibrium is dP/dr = -GM(r)ρ/r^2 and as for your "conservation of mass" equation, if it is the one I think it is, if you don't know it then all you need to do is think about how you would get the mass of a spherical shell. HTH.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook