# Answer to Question #2688 in Molecular Physics | Thermodynamics for Eman Faniin

Question #2688

& A hot-air balloon must support its own weight plus a person for a total weight of 1200 N Ambient air is at 20C and 1 atm . The hot air inside the balloon is at 65C and 1 atm. What diameter of spherical balloon just will support the total weight ? neglect the size of the hot-air inlet vent .

Expert's answer

Archimedes' theorem: the ball will support the total weight in the air, if the density of the ball with air + cargo equal to the density of air at 20 degrees.

1200 N = 120 kg

ρ

ρ

ρ

thus

V = 755 m

Hence diameter D = 2r = 11.3m

1200 N = 120 kg

ρ

_{sys}= (120 +V* ρ_{ 65})/V = ρ_{20}ρ

_{65}= 1.045 kg/m^{3 }- density at 65 degreesρ

_{20}= 1.204 kg/m^{3}- density at 20 degreesthus

V = 755 m

^{3 }=> r = (3V/ 4 π)1/3 = (180) 1/3 = 5.65 mHence diameter D = 2r = 11.3m

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