Conceptual Physics (12th Edition)

Published by Addison-Wesley
ISBN 10: 0321909100
ISBN 13: 978-0-32190-910-7

Chapter 6 - Think and Explain: 54

Answer

The blades need to push air downward to provide an impulse to the air, i.e., a downward change in the air's momentum. Newton's third law tells us that the air acts with an equal and opposite force on the blades. This acts for the same time, giving an upward impulse on the blades, providing an upward change in momentum for the helicopter (which we call "lift"). In other words, because the interaction times are equal, Newton’s third law guarantees equal and opposite impulses on the 2 objects, not just equal and opposite forces.
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