A balloon of weight w is falling vertically downward with a constant acceleration $\mathrm{a}\left(<\mathrm{g}\right)$ , The magnitude of the air resistance is

Suppose air resistance is F (upwards), then from equation of motion of balloon, we have 

$$\begin{gathered} \mathrm{w}-\mathrm{F} =\mathrm{mass} \times \mathrm{acceleration}=\frac{\mathrm{w}}{\mathrm{g}}.\mathrm{a} \\ \therefore \mathrm{F}=\mathrm{w}\left(1-\frac{\mathrm{a}}{\mathrm{g}}\right) \end{gathered}$$