A party balloon(of mass 0.0025kg when empty) is filled with helium to a volume of 0.0045m3. it is tied to a small stone of mass 0.015kg by a light string of length 1.5m to prevent it from flying away. a child holds the balloon at ground level and then releases it.
(a)how long does it take for the balloon to rise 1.5m,that is,for the string to become taut?
(b)What is the velocity of the stone when it is lifted off the ground? you may assume the time needed to bring the stone to this velocity is very short(impulse approximation) once the string is taut, and the string remains taut throughout
(c)How long after lifting off will the stone touch the ground again?(neglect air resistance and treat the stone as a particle)
2. The attempt at a solution
part 1 is relatively easy,i have mtotal=0.00330kg,net upward force=0.0206N,a=6.24m/s2, t=0.693s which agrees with the given answer
not really understand part 2 as the net upward force acting on balloon is only 0.0206N,how can it then lift a stone with a weight of 0.147N?
for part 3,if the stone can be lifted,why would it touch the ground again?