October 22, 2020

Can You Stop a Car With Your Hand?

“When I stick my arm out a car window, I can feel the resistance of the air. Could I slow the car down this way, eventually? How many arms would I need to brake effectively?”

— Jean, Lyon, France

Sticking your hand out the window on the highway produces a few pounds of drag on the car. The exact amount depends on how you shape your hand; spreading out your fingers creates a little more drag than holding them together.

A few pounds of drag is easy to feel on your arm, but it has a fairly small effect on the car. Imagine your car is cruising along at 55 miles per hour, with the engine providing just enough power to balance out the normal air resistance and other types of mechanical friction and keep your speed constant. If you then upset the balance by sticking your hand out the

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