I'm trying to calculate the acceleration of a high altitude balloon at different altitudes. In excel I've got the temperature, air pressure, and air density for a given altitude (thanks to NASA). Right now I am calculating the net force on the balloon with: Fnet= Fb-Fm Fb = Buoyant force = density of air*volume*9.81 Fm= (mass of balloon+mass of payload+(volume*density of helium))*9.81 For whatever volume I put in for the balloon I only get a positive net force until about 55,000 feet. I'm thinking it is because I am not taking into account the increase of air that will be displaced by the balloon as it gets higher and higher, but im not sure how to model that mathematically. Any insight as to how to do this would be appreciated.