OK, I have been working on this for a while. I have a balloon, filled with helium. mass = 140 kg density(air) = 1.16 kg/m^3 density(He) = 1/7*density(air) diameter of balloon = 10 m bouyant force = density(air)*g*Volume(balloon) weight = m*g net force = bouyant force - weight = ma I need to solve for a. The book tells the answer (16.5 m/s^2), so I know what I need to get to, but doing the steps above, I haven't gotten close. Anybody have any ideas to help out? Thanks!