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f = ma

f = - GMm/r^2

a = -GM/r^2 can be easily derived,

But, we've been given the differential equation of motion of gas within a star as:

a = -GM/r^2 - 1/p * dP(r)/dr

I was wondering where the - 1/p * dP(r)/dr term is derived from? I can't find it in my text books.

Cheers

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# Differential equation for motion within stars

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