back story . i bombed my midterm . and i get a chance to make up some marks by redoing it at home . but i'm stuck on one question . so here goes . the planet mars has two satellites, Phobos and Deimos, that orbit mars along almost circular paths . a) Phobos average orbital radius [R] is 9.378 * 10^6m , and it's orbital period [T] is 2.754 * 10^4s. Calculater Mars's mass . b) Deimos's orbital period is 1.08 * 10^5s . Calculate the ratio of the satellites orbital speeds (Vp/Vd) . ---- so the first thing i did was get the acceleration with a=(v^2/r) . so a=4.95*10^-3 . my thought was that i was going to use F=(G*M1*M2)/R^2 to get the F , and then use that to get the mass . but obviously i forgot that i don't have one of the masses . so that won't work . and i know that for b) the velocity[v] = 2?r/t . but then i only have the orbital radius for Phobos . so i have: Rp = 9.378*10^6 Tp = 2.754*10^4 Td = 1.08*10^5 the centripetal acceleration [a] of mars and phobos = 4.95*10^3 Vp = 2.139*10^3 now from here i'm stumped . i have all sorts of crap , and i'm still missing the mass of mars and one of the velocities . so if anybody out there had a hint or two they could give me , i would be forever gratefull . maybe i'll even shovel your walk in the winter , or rake your leaves in the fall .