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**1. Homework Statement**

Calculate the escape velocity on the surface of the neutron star in the previous problem (##m = \frac{2}{3} \cdot 2,1 \cdot M_{\odot}##; ##R = 15km##).

Hint: Basic physics. Note, however, that the escape velocity is not going to be small when compared to the speed of light.

**2. Homework Equations**

Relativistic kinetic energy:

\begin{equation}

K = \gamma mc^2 - mc^2 = \sqrt{(pc)^2 + (mc^2)^2} - mc^2

\end{equation}

Standard Newtonian potential energy:

\begin{equation}

V_N = -G\frac{mM}{r}

\end{equation}

Relativistic potential energy:

\begin{equation}

V_R = ?

\end{equation}

**3. The Attempt at a Solution**

My idea was to set the kinetic and potential energies to be equal, and solve for the speed as usual. The problem is, I don't know (how to derive) the expression for a relativistic potential, which I'm probably going to need because, you know, it's a neutron star we're talking about here.

I found a source (since none of my books were of any use), that simply multiplies the standard Newtonian potential with the Lorentz-factor ##\gamma##, but I'm not sure that's allowed.

Is it, and if so, why?