hi all just doing some questions and come along these two which have me stumped. There multiple choice questions but id rather know the working out to them as i have exams in a couple of weeks. thanks. 1. The mass of a hot-air balloon and its fully loaded basket is 500kg excluding the air in the balloon. if we assume the balloon is spherical, what radius must the ballon be for it to float if the density of the hot air inside the balloon is 0.95kg/m^3 and the density of the cold air outside is 1.3kg/m^3. --- with this question i belive you use the P = Po + pgh but i cannot set it out to find the radiusi have tried to use 4/3 pi R^3 where m = pV. 2. A sphere of mass 1.9g & diameter 8mm made of steel is dropped into a tank of liquid with density = 900kg/m^3 and falls a distance of 30cm in 27 seconds. What is the Viscosity of the oil? --- i looked through text books but i could only find newtons law for viscosity and i had no idea how to work it out. and just one quick check i believe this is right. Newtons law of viscosity says that: - the stress depends on the acceleration( i belive this is it because the equation is stress is the viscosity of the liquid x the change in velocity i believe?) the other options for the answer were 1. the stress is proportianal to the rate of strain and 2. The velocity gradient is linear. but i see no relation with these other 2 answers. any help is much appreciated thanks!