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Corsair
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This is actually a problem in Goldstein.
A rocket that ejects stuff at a speed a in its rest frame. Demonstrate that
[tex]m\frac{d v}{dm} + a\left(1 - {v^2 \over c^2}\right) = 0[/tex]
in which m is the invariant mass of the rocket and v is the velocity of the rocket viewed in Earth frame.
First work in rocket rest frame. At some time, the 4-momentum of the rocket is
[tex]p_0^\mu = (mc, 0)[/tex]
After dt time, the 4-momentum of the rocket is
[tex]p_r^\mu = \left((m-dm)c ,\ m\,dv\right)[/tex]
I ignored [tex]dv^2[/tex] and [tex]dm\,dv[/tex]. Also the stuff the rocket ejects during this time is
[tex]p_g^\mu = \left(\delta m(c + \frac{1}{2c} a^2), \ \delta m\, a\right)[/tex]
Since the equation in the problem has v, I have to boost those vectors into Earth frame. And momentum is conserved, therefore
[tex]\left(\begin{array}{cc}1&-\beta \\ -\beta &1\end{array}\right) p_0^\mu = \left(\begin{array}{cc}1&-\beta \\ -\beta &1\end{array}\right) p_r^\mu + \left(\begin{array}{cc}1&-\beta \\ -\beta &1\end{array}\right) p_g^\mu[/tex]
But what I get is a very messy and long stuff that has no way to be the same as the equation in the problem. So where did I do wrong?
Homework Statement
A rocket that ejects stuff at a speed a in its rest frame. Demonstrate that
[tex]m\frac{d v}{dm} + a\left(1 - {v^2 \over c^2}\right) = 0[/tex]
in which m is the invariant mass of the rocket and v is the velocity of the rocket viewed in Earth frame.
Homework Equations
The Attempt at a Solution
First work in rocket rest frame. At some time, the 4-momentum of the rocket is
[tex]p_0^\mu = (mc, 0)[/tex]
After dt time, the 4-momentum of the rocket is
[tex]p_r^\mu = \left((m-dm)c ,\ m\,dv\right)[/tex]
I ignored [tex]dv^2[/tex] and [tex]dm\,dv[/tex]. Also the stuff the rocket ejects during this time is
[tex]p_g^\mu = \left(\delta m(c + \frac{1}{2c} a^2), \ \delta m\, a\right)[/tex]
Since the equation in the problem has v, I have to boost those vectors into Earth frame. And momentum is conserved, therefore
[tex]\left(\begin{array}{cc}1&-\beta \\ -\beta &1\end{array}\right) p_0^\mu = \left(\begin{array}{cc}1&-\beta \\ -\beta &1\end{array}\right) p_r^\mu + \left(\begin{array}{cc}1&-\beta \\ -\beta &1\end{array}\right) p_g^\mu[/tex]
But what I get is a very messy and long stuff that has no way to be the same as the equation in the problem. So where did I do wrong?
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