# Help! Conservation Of Mass

1. Mar 20, 2005

### GotTrips

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

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

Thanks All

2. Mar 20, 2005

### Nylex

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.