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physics question

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?

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2 Answers

In order to hover, the helicopter blades force 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 x final air speed

Equate this to the force of gravity on the helicopter to find the air mass flow rate.