Q 11-04-156JEE MainJEE Main 2022 (28 Jul, Shift 1)Easy
A balloon has mass of $10$ g in air. The air escapes from the balloon at a uniform rate with velocity $4.5\ \text{cm s}^{-1}$. If the balloon shrinks in $5$ s completely. Then, the average force acting on that balloon will be (in dyne).
Answer: (B) $9$
Rate of mass loss: $\dfrac{dm}{dt} = \dfrac{10\ \text{g}}{5\ \text{s}} = 2\ \text{g s}^{-1}$.
Thrust on the balloon:
$$F = v\frac{dm}{dt} = 4.5\times2 = 9\ \text{dyne}$$
Solution by Sreeraj P, M.Sc Physics