Finding net force and acceleration of a 10 m diameter helium balloon

[physicsCU]

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
buoyant force = density(air)*g*Volume(balloon)
weight = m*g
net force = buoyant 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!

 

[Galileo]

You need the mass of the 'entire' balloon, that is, the mass of the container itself which is 140 kg, plus the mass of the helium inside. I think that's about the only point where it can really go wrong.
Why don't you post what you got for the volume and mass of the balloon and what you got for the bouyancy.

 

[physicsCU]

ok, volume was 523.6 m^3, mass ended up being 226.768 kg.

i had forgot the mass of the helium, i got the right answer now, thanks!