For electron-positron interactions, we have different cross sections, depending on the available reaction energy. To get an overview how many particles of a certain type have been created, we can calculate cross section ratios, f.ex. \begin{equation} R_{had}=\frac{\sigma \left ( e^-e^+ \rightarro...

I was shown this derivation for the gravitational potential energy, and I'm not very happy about it assuming that $\frac{1}{\infty} = 0$. Is there a better derivation, either using a completely different method, or one similar that avoids $\frac{1}{\infty}$? \begin{align} \text{work done} &= \int...

Of course, one can cover circular motion with different speed at different position, right? The only aim is to complete his circular motion, right?