My physics problem is as following, Estimate the average temperature of the air inside a hot air balloon. Assume that the total mass of the unfilled balloon and payload is 500kg. what is the mass of the air inside the balloon? we are told to use a radius of 15m Equations needed density = m/V ideal gas law PV=nRT Pressure = Force over area. This is what i Have so far: Fnet = Fbuoyant - Fgravity because the balloon is assumed to be in equilibrium Fnet = zero 0= density*V*g -(mass of air+mass of payload)*g Its here that i start to get confused. I think that the density is the density of the air alone because it is the buoyant force of the heated air pushing up on the balloon so to speak. I know that there is a relation to the gas law and from that the temperature of the balloon can be calculated....but i am stuck at this: P= density*R*T any suggestions/help?