During a rescue operation, a 6000-kg helicopter hovers above a fixed point. The helicopter blades send air downward with a speed of 65 m/s. What mass of air must pass through the blades every second to produce enough thrust for the helicopter to hover?
In order to hover, the helicopter blades forces air downward so that the reaction force on the helicopter balances the force of gravity. The magnitudes of the action-reaction force pair are equal and since the force on the air changes the air's momentum from rest to the final speed,
F = mass flow rate * final air speed
Equate this to the force of gravity on the helicopter to find the air mass flow rate.