“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 freeway produces a number of kilos of drag on the automobile. The actual quantity depends upon the way you form your hand; spreading out your fingers creates a little bit extra drag than holding them collectively.
A number of kilos of drag is simple to really feel in your arm, nevertheless it has a reasonably small impact on the automobile. Imagine your automobile is cruising alongside at 55 miles per hour, with the engine offering simply sufficient energy to steadiness out the conventional air resistance and different sorts of mechanical friction and hold your velocity fixed. If you then upset the steadiness by sticking your hand out the window, including a little bit extra drag, it might take a minute or two for that drag to trigger your velocity to fall from 55 miles per hour to 54 miles per hour.
The brakes on a typical automobile can produce a number of thousand kilos of deceleration drive, which is the equal of 1,000 or so arms protruding of the automobile. I’m undecided how you can get that many arms protruding the home windows without delay. If the arms are connected to individuals, then I’ve questions on your seatbelt and occupancy state of affairs. (If the arms aren’t connected to individuals, I’ve some greater questions.)
In place of 1,000 normal-size arms, you can strive utilizing one or two very giant arms, maybe a pair of these big foam arms utilized by sports activities followers. At 55 miles per hour, two 15-foot-long arms would produce roughly as a lot drag because the automobile’s regular brakes.
Using drag for braking works significantly better at excessive speeds than low speeds. Slower-moving air creates much less drag, in order you decelerate, you would want even greater arms to supply the identical braking drive. At 45 miles per hour, you would want 20-foot-long arms to get the identical braking drive as 15-foot arms at 55 miles per hour. At 30 miles per hour, you would want 30-foot arms. At 15 miles per hour, every hand would have to be 60 ft lengthy, making your automobile too huge to suit on most roads.
Drag is extraordinarily weak at low speeds, so it might be laborious to come back to an entire cease. A headwind might make it easier to come to a relaxation extra rapidly, however you’ll be helpless if you happen to have been touring downwind. To cease, you would possibly discover that need to resort to flapping the arms like paddles or dragging them on the bottom.
One notice of warning: If you do construct these brakes and drive on the freeway, watch out to not tilt your arms ahead.