1. It is problem for kibble book, classical mechanics. I star with mass M traves through cloud of particles with density ro. Particles that collide with a star are trapped by it. Derive the expression for mass increase of the star dM/dt. 3. The attempt at a solution Since all particles that collide with a star are trapped it's contribution is ro*Vstar*(piR^2) However the particles that that are outside of cross sectional area are also get trapped by it. due to gravitational attraction. Thats where I had hard time to writing down conditionsat. I was thinking of trying to figure out understand simmiar problem, suppose object of mass m, starts at (x=infinity, y=b) traveling with velocity V towards s star of mass M. So, what is the how far from the star an object should travel to get trapped by it into an orbit, but could not answer it. Well any even smallest hint on how to solve the problem will be greatly appretiated. I have a feeling that problem is quite simple but answer eludes me.