A factor of 2 is not an approximation!
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Originally Posted by GregT
And he just approximated 
Should've used Dalton's law to solve for moles of the gas components (taking into account the composition of your local air supply) then used Van der Waals modified ideal-gas equation to calculate the partial weights of each gas then added them all together. The bouyant force's effect on the balls weight force is easy to take into account if your assuming 2 ATM's pressure in the ball- just devide your final air weight by two (keeping in mind the air mass stays the same... for all those acceleration problems).
Don't forget molecules have volume and experience forces between eachother!
Of course even this is an approximation! You would need an even more complex model to take into account the molecular attaction between the differrent types of molecules in air.
Sorry about the extremely unecassary sarcasm there I got very (_very_) close to your answer by doing it my way. (good job)
Does anyone find it a bit concerning that we are trying to take the air's weight of into account? A gust of wind or well placed vent could easily contribute that much force. I think we've determined the ball is in the 3 - 6 lb range, if I were designing a mechanism I would assume 10 lbs.
(good luck on that test dan)
Greg |