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Helium Ballon

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A helium ballon has volume V0 and temperature T0 at sea level where the pressure is P0 and the air density is p. The ballon is allowed to float up in air to altitude y where the temperature is T1.

a) Show that the colume occupised by the ballon is then V = V0(T1/T0)e^(+cy) where c = pg/P0 = 1.25 X 10^4 m^-1.

b) Show that the buoyant force does not depend on altitude y. Assume that the skin of the ballon maintains the helium pressure at a constant factor of 1.05 times greater than the outside pressure. [Hint] Assume that the pressure change with altitude is P = (P0)e^(+cy).

I was able to use the hint to do a), but I'm confused by b). Why does the book say to assume he skin of the ballon maintains the helium pressure at a constant factor of 1.05 times greater than the outside pressure?
 

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