Archimeded problem

[kindaichi]

I can't really understand Archimedes principle..I mean i know how it works in water but i am totally clueless when it comes to air question.Here is a question:

A balloon having a mass of 120 kg and volume of 400mcube is filled with helium gas of density 0.18kg m to the power of -3.The density of the surrounding air is 1.28kg m to the power of -3.What is the maximum load that can be lifted up by the balloon?Answer:320kg

How do you solve this question?Also can someone please tell me how to count air question..My exam is a few days away:confused:

[semc]

dude draw the FBD of the balloon the lift force is pgv where p is density of air,g is 9.81 and v is volume of air displaced or u can say volume of balloon. i think u can find the ans from there right?

[kindaichi]

I keep getting 392kg whereas the answer is 320kg.I have been spending an hour here

[Doc Al]

Don't forget to consider the mass of the helium itself.

[kindaichi]

so basically,you count the buoyancy force first and then later subtract the weight of the balloon and helium gas?

[semc]

hmm.....i got exactly 320kg maybe calculation error?

[Doc Al]

so basically,you count the buoyancy force first and then later subtract the weight of the balloon and helium gas?Right. Whatever is left over is the additional load that can be lifted.

[kindaichi]

oh,i see then..thank you everyone.Everyone have been a great help
Just,one last question.The force acting downwards is the buoyancy force right?