Today: Tom and Ray settle a confounding physics conundrum.

Dear Car Talk

Dear Car Talk | May 01, 2009

Dear Tom and Ray:

Once, on a very long, boring trip, my passenger and I were reduced to asking each other dumb questions. We came upon one question that we couldn't settle. Imagine that I'm driving along at highway speed with the air conditioner on and the windows closed. A hummingbird that was caged in the back seat gets loose, and with nowhere really to go, it ends up just hovering there in midcar. Our question is this: If I hit the brakes hard, does the hummingbird crash into the windshield? My friend said of course it would, that its momentum would cause it to keep moving forward as the car slowed. But I, being the more educated of us (not necessarily a good thing, as my friend claimed), said that the hummingbird's position would depend on its air speed, not its ground speed, and as the car slowed down, the air inside the car would slow down at an equal rate, as would the hummingbird, thereby avoiding becoming windshield splatter. So guys, please settle this question so I can finally get some sleep.

-- Ross

TOM: Why don't you just put the darn hummingbird back in its cage, Ross?

RAY: As your friend says, Ross, over-education might not always be a good thing. But when you work around my brother, you come to the inescapable conclusion that a little bit more education can never hurt.

TOM: The answer is that the hummingbird is toast, Ross. He crashes into the windshield. As your friend correctly says, in summarizing Newton's first law, objects in motion will stay in motion.

RAY: The cabin air is part of the car. So if the hummingbird is hovering in that air while the car is in motion, the bird has the same horizontal speed as the car.

TOM: That means he's in motion. And when the car stops, the hummingbird does not, and splat.

RAY: Now, as we were discussing your question, Ross, my brother raised an interesting issue.

TOM: That's right. I said, "What about cigar smoke?" Let's say I'm motoring along in one of the new cars that we test-drive, smoking a huge cigar.

RAY: Of course, this is a hypothetical question, because you're not allowed to smoke in the test cars.

TOM: I'm not? Oh! I'm not! Right. But let's say, just for the sake of argument, that the smoke is hovering in the air all around me, and I stop short. Does the smoke crash into the windshield? I don't think so.

RAY: It's a good question. So we called in our Car Talk physicist, professor Wolfgang Rueckner, who also moonlights at Harvard University.

TOM: Wolfgang says that the smoke, too, is subject to Newton's first law, and should crash into the windshield. The only reason it doesn't is because it doesn't have enough mass. So while it's heading toward the windshield, it bangs into nearby air molecules, and the effect is muted.

RAY: I'm wondering if Newton ever tried this physics experiment: Take a test car that my brother has been smoking a cigar in, and have the manufacturer who owns it stick his head inside and take a sniff. Then see if that guy slams my brother's head into the windshield.

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