Question 20N.2.SL.TZ0.a(i)

A company delivers packages to customers using a small unmanned aircraft. Rotating horizontal blades exert a force on the surrounding air. The air above the aircraft is initially stationary.

The air is propelled vertically downwards with speed $v$. The aircraft hovers motionless above the ground. A package is suspended from the aircraft on a string. The mass of the aircraft is $0.95\text{\hspace{0.05em}kg}$ and the combined mass of the package and string is $0.45 \mathrm{kg}$. The mass of air pushed downwards by the blades in one second is $1.7 \mathrm{kg}$.