A balloon has mass of 10   g in air. The air escapes from the balloon at a uniform rate with velocity 4…

Given that the mass of the air $m=10 \mathrm{g}$ and it escapes in time $t=5 \mathrm{s}$ with velocity $v=4.5 \mathrm{cm} {\mathrm{s}}^{-1}$.

The average force on the balloon can be given as,

$F=v\frac{dm}{dt}$

$\Rightarrow F=4.5\times \frac{10}{5}=9 \mathrm{dyne}$