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How does a helicopter obtain the upward force that lifts it?
ABy pushing air sideways and feeling an upward reaction
BBy reducing its own weight as the rotor turns faster
CBy pulling air upward and feeling an upward reaction
DBy pushing air downward and feeling an upward reaction
Answer & Solution
Correct answer: D. By pushing air downward and feeling an upward reaction
1. The rotor of a helicopter drives air downward.
2. By Newton's third law the air pushes back on the machine with an equal upward force.
3. That upward reaction force is the lift that supports the helicopter.
4. Birds and aeroplanes fly on the same principle, forcing air in the direction opposite to the force they need.
5. Nothing about the rotor changes the weight of the aircraft.
_Source: OpenStax College Physics (CC BY 4.0), Ch 4 "Dynamics: Force and Newton's Laws of Motion", section 4.4 Newton's Third Law of Motion: Symmetry in Forces_
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