I currently have the topic Newtonian gravity, which is described as a field theory by means of the Poisson equation $$\Delta \phi = 4\pi G\rho\,.$$ As an assignment I have: derive the equation of motion for test masses. (1 point, short task)
I can do something with the single words, but their combination to this sentence, I don't understand. What should I derive? And from what? Should I derive according to the mass, so $\frac{d}{dm}$? And why? What is the point of that? Where should this lead me. This task confuses me.