Today: Tom and Ray ponder the now-famed "hummingbird" conundrum.

Dear Car Talk

Dear Car Talk | Jun 01, 2010

Dear Tom and Ray: In a recent column, you got the wrong answer to the "hummingbird" problem. Fifty years ago, my boss in an Air Force research laboratory used this problem to trick those who wanted to upgrade their "subprofessional" status to "professional." He used a balloon instead of a hummingbird, but the physics is the same. The air conditioner's operation is irrelevant, but the closed windows are key. In the closed environment, when the brakes are applied, the air rushes to the front, forming a pressure gradient that pushes suspended objects to the rear. Make MIT proud and admit a mistake. -- Ed

TOM: Gee, Ed. Our alma mater is already busting its buttons from all of our mistakes. But I don't think this is one of them.

RAY: The problem was this: You had a hummingbird flying in the middle of a car that was traveling down the highway with the windows closed. The driver slams on the brakes, and the question was, Does the bird crash into the windshield?

TOM: You're right that a balloon would go backward, but the physics ISN'T the same for the hummingbird. The balloon stays aloft only because of the buoyant force of the air pressure in the car.

RAY: The hummingbird stays aloft for a very different reason: Newton's third law -- the old "equal and opposite reaction" thingy. The bird is using its wings to force air down, which creates an equal and opposite force of air pushing up, allowing it to hover in place.

TOM: Unlike the air in the car that's keeping the balloon afloat, the bird's mechanism doesn't change when the car stops short.

RAY: And Newton's first law tells us that objects in motion (the hummingbird is moving at the same speed as the car) stay in motion, unless something gets in their way.

TOM: Like the windshield.

RAY: Don't feel bad, Ed. It's a complicated problem. And if you keep reading, you'll undoubtedly find something else to correct us on soon enough.

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