CLICK & CLACK : A Real Humdinger

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Sunday, May 17, 2009

Dear Tom and Ray:

Once, on a very long 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 it ends up just hovering there in mid-car. 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 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. So, guys, please settle this question. -- Ross

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, my brother raised an interesting issue.

TOM: 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. Let's say 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: 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.

Dear Tom and Ray:

My wife and I recently bought a 5-speed Saturn VUE. Whenever she comes to a complete stop, she puts the car in second gear and takes off in second gear. She says it is better for the car, and that's the way her dad taught her to do it. I say that it isn't better for the car, because it forces the engine to spin at higher RPMs, putting more wear and tear on the engine and clutch. I know I will lose this argument no matter what, because dads always know best. But I'd still like to know what you think. -- Jeff

TOM: Jeff, we take no responsibility for what happens should you use this information in an actual family setting. But here it is.

RAY: You're 100 percent right; her dad is 100 percent wrong. The thing that wears out a clutch more than anything else is starting off from a dead stop. That requires the greatest amount of gas and the slowest release of the clutch pedal. And that combination -- gas and time -- is what kills clutches.

TOM: When starting in second gear, you have less of a mechanical advantage than you do in first gear. That means you have to compensate by giving the engine more gas and letting your foot off the clutch even more slowly to avoid stalling.

RAY: By comparison, the other shifts -- from first to second, second to third, etc. -- cause almost no wear and tear on the clutch, because the car is already moving.

Got a question about cars? Write to Click & Clack in care of The Post, or e-mail them by visiting the Car Talk Web site at http://www.cartalk.com.

© 2009 by Tom and Ray Magliozzi and Doug Berman


© 2009 The Washington Post Company

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