# 1st year physic help - hot air balloons

1. Nov 6, 2005

### bruceflea

1st year physic help - hot air balloons....

see next post...

Last edited: Nov 6, 2005
2. Nov 6, 2005

### bruceflea

question:

The buoyancy force that pushes the balloon upward is proportional to the density of the cooler air outsider the balloon and the volume of the balloon, and can be expressed as

force buoyancy = denisty(ofcoolair)*g*volume(ofballoon)

where g is the gravitational constant.

Consider a 20-m-diameter hot-air balloon that, together with its cage, has a mass of 80 kg when empty. This balloon is hanging still in the air at a location where the atmospheric pressure and temperature are 90 kPa and 15 o C, respectively, while carrying three 65 kg people. Determine the average temperature of the air in the balloon.

this is what I have so far...

The buoyancy force required for the balloon to hang still in the air is equal to the force exerted downwards by the weight of the balloon so:

Force buoyancy, Fb = mass*gravity = 275*9.81 = 2697.75 N
Therefore:

denisty(ofcoolair)*g*volume(ofballoon) = 2697.75 N

volume(of balloon) = 4/3*pi*10^3 = 4188.79 m^3

density(ofcoolair) = 0.066 kg m^3

I know that for the balloon to be hanging still:

density(ofcoolair) = density(ofhotair)

Is it ok to assume that the pressure inside the balloon is the same as the outside (90kPa)? I'm also assuming that V1 = V2. Which would give me the expression:

T2/T1 = n1/n2

Am I heading in the right direction?

Last edited: Nov 6, 2005