1. The problem statement, all variables and given/known data How many cubic metres of helium are required to lift a balloon with a 400 kg payload to 8000 m? Assume balloon maintains constant volume, and density of the air decreases with altitude z according to the expression [tex]\rho[/tex]air = [tex]\rho[/tex]^e^-z/8000, where z is in metres and po = 1.25 kg/m^3 is the density of air at sea level. 2. Relevant equations [tex]\rho[/tex]o = [tex]\rho[/tex]fluid(g)h B = [tex]\rho[/tex][(g)V [tex]\rho[/tex][ = m/v mg < [tex]\rho[/tex](g)V (Not sure if this is right?? But buoyancy should be stronger if it is to rise, correct?) I'm not sure how to go about this. I see I need to find volume, so I know the Buoyancy equation with be used, and I think I should find the Buoyancy-- so I assume I need to find the Buoyant force on the "payload" (whatever that is.) Since I don't know the payload's density or volume, I figure that probably mg = [tex]\rho[/tex](g)V (400)(9.81) = B = [tex]\rho[/tex](g)V But this doesn't seem to make sense to me??