A lighter-than-air spherical balloon and its load of passengers and ballast are floating stationary above the earth. Ballast is weight (of negligible volume) that can be dropped overboard to make the balloon rise. The radius of this balloon is 6.59 m. Assuming a constant value of 1.29 kg/m3 for the density of air, determine how much weight must be dropped overboard to make the balloon rise 116 m in 17.0 s. Hi, that's a physics problem aht confounds me. It's about archimedes principle (weight of fluid displaced=bouyant force) , right? I've found out the bouyant force using that principle. And the part when the baloon moves can be solved by doing (bouyant force-weight of the balloon=Resultant force) wherein the resultant force causes the balloon to rise up(resultant force=mass of balloon*vel (116-17)). Is that assumption right? if i do that i get the new mass of the balloon but i don't know how to get it's intial mass and i need it to find out how much mass needs to be dropped. Can anyone help me with my problem please?