 #1
Jpyhsics
 84
 2
 Homework Statement
 A hotair balloon is initially floating (stationary) a few metres above the ground. The radius of the spherical balloon is 6.25 m, and the density of surrounding air is 1.29 kg m3. "Ballast" is weight of negligible volume that can be dropped to make the balloon rise. How much weight would you have to drop overboard to make the balloon rise 105 m in 15.0 s? You may assume that the density of surrounding air remains constant.
 Relevant Equations

Archimedes principle: Fb=rho*gV
Vsphere= 4/3*pi*r^3
Initially the hot air balloon is stationary so...
F_{B}=F_{g}
ρgV=mg
m=1319.2...kg
F_{B}=F_{g}=12941N
In the air...
a=2d/t^2=0.933...m/s^2
F_{net}=F_{B}F_{g}
(1319x)(0.93...)=12941(1319x)g
x=114.6 kg
but apparently this is wrong?...
F_{B}=F_{g}
ρgV=mg
m=1319.2...kg
F_{B}=F_{g}=12941N
In the air...
a=2d/t^2=0.933...m/s^2
F_{net}=F_{B}F_{g}
(1319x)(0.93...)=12941(1319x)g
x=114.6 kg
but apparently this is wrong?...