Conceptual Physics (12th Edition)

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

Chapter 4 - Think and Explain - Page 71-72: 86

Answer

You run your engine for the same reason that you have to push a crate to keep it sliding at constant velocity. There are friction and fluid drag forces acting on your car, and your engine's force must offset them to keep the net force equal to zero. The net force is zero because your car moves at constant velocity. This is discussed on page 60.
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