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Homework Statement
I'm considering an expanding spherical shell of gas in the thin shell approximation, in which all of the mass can be considered to be at radius R. This shell encloses a bubble that has been blown out (by a supernova or something) and is expanding into the ambient interstellar medium (ISM), picking up mass along the way, like a snow plow. The simplest approximation would be that the shell mass M(R) at a given radius R is given by the total ambient mass of ISM material that was once enclosed by that radius. However, I do not think that I am supposed to assume any specific functional form for M(R) for the first part of this problem. I'm told also that the shell has some total force on it (which can consist of both inward and outward forces) given by F(R), which is some known function. The problem asks me to :
"Use the fact that F is the rate of change of the shell momentum to derive v(R) = dR/dt. Then show how R(t) can be derived from v(R). You do not have to plug in the actual v(R) in the second step."
Homework Equations
[tex] \textbf{F} = \frac{d\textbf{p}}{dt} [/tex]
The Attempt at a Solution
So far I have written:
[tex] F(R) = \frac{d}{dt}[ M(R) v(R) ] = M(R)\frac{dv(R)}{dt} + v(R)\frac{dM(R)}{dt} [/tex]
[tex] F(R) = M(R)v^{\prime}(R)\dot{R} + v(R)M^{\prime}(R)\dot{R} [/tex]
[tex] F(R) = M(R)v^{\prime}(R)v(R) + M^{\prime}(R)v^2(R) [/tex]
[tex] F(R) = M(R)v^{\prime}(R)\dot{R} + v(R)M^{\prime}(R)\dot{R} [/tex]
[tex] F(R) = M(R)v^{\prime}(R)v(R) + M^{\prime}(R)v^2(R) [/tex]
A differential equation which I have no idea how to solve. I'm thinking that I may be wrong about the expression for the total shell momentum just being Mv. After all momentum is a vector, and the total shell momentum should be the sum of the momenta of the individual particles comprising it. My thinking is that, in the absence of F(R), the total momentum is zero (in the rest frame of the object that exploded), and this can be shown simply by pointing out that for every bit of mass that is flying outward radially in some direction, there is another bit of the same mass that is flying radially outward in the opposite direction (by symmetry). However, another side issue that is also confusing me is that, using spherical coordinates, the particles of the gas all have momenta in the [itex]+\hat{\textbf{r}} [/itex] direction, which would lead one to state that there is some net momentum in the "radially outward" direction. I understand that the [itex]\hat{\textbf{r}} [/itex] vector changes direction, but I'm still confused. What is wrong with that statement?
Any help on both the problem itself and the side issue I identified would be much appreciated!