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#0823: Lillian and the Thousand Dollar Valve

Original Air Date: 06.07.2008

   Best Moment

Is this Car Talk or "60 Minutes"? This week, 83-year old Lillian turns to Tom and Ray to help decipher a suspicious diagnosis from her usually trusty mechanic. Does he really want to charge her a thousand bucks to replace a valve? Has he succumbed to the beginning of boating season? You won't want to miss it, as our hosts go to the source to try to get the truth. Meanwhile, our Physics advisor Wolfgang is on-hand to administer a (well-deserved) dope slap to the boys for their (wacko) theory on head-on collisions. Also, what do you do when your Maxima's windows go cuckoo? If you're Adam in Baltimore, blame your girlfriend! All this, plus Tommy communes with nature and winds up paying for it in our new Puzzler, and lots more, this week on Car Talk.

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This Week's Puzzler

Tommy spills a basket of eggs. What's the smallest number possible? Find out!

Last Week's Puzzler

From Ray's dining and dancing series: why was he still hungry, even after going to the restaurant? Find out!

Show Open Topic

How you know when it's time to trade in your car, part two.

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You need a new professor

by Anonymous

Disregarding any theories about elasticity of the wall or the other car and assuming all things created equal (perfect, elastic collision) just to make the math easy, we'll examine why you need a new professor. And I hate to say this because I had some excellent physics professors, but you should find an engineer. Ok, I didn't hate it that much... In using the momentum (P) as the basis of Wolfgang's theory, he failed to realize that momentum of a system of particles (i.e. car A and car B) is the vector sum of all the momenta. Forces will be equal regardless of your frame of reference, therefore if we hold car B stationary, car A will appear to be oncoming at twice the original speed with twice the momentum. This problem has to be evaluated as a system of both vehicles and can't be simplified to just one vehicle. The energy balance doesn't work if you remove the second vehicle. Your original evaluation was correct: The force of car A (with xmass and yvel) impacting car B (also with xmass and yvel) is equal to car A (with xmass and 2*yvel) impacting an immovable wall as seen from car A.

This show runs worse than my car

by rickahyatt

I try to download the "Watch a clip" (Where's the option for the whole show?) and a little box appears and a giant "Q" highlights and goes dim, over and over again, saying something about have went somewhere and then STOPPED. Worse than my car.

Favorite Moment: Will be when the damn thing runs!


by Anonymous


Favorite Moment: oinoi

The Harvard Physics Prof was WRONG!

by dsiegfried

The Prof was WRONG!!! As an engineer, I can with some trepidation, absolutely state that the Prof was wrong. It does matter if it's a wall or a car with equal momentum. The question is "how strong is the wall? If it's the rock of Gibralter, maybe he's right. But when he said they are equal due to equal amounts of momentum being lost (mv=0 at the end), he's wrong. If I apply my brakes and slowly stop the car, the momentum lost is equal to the collision if both vehicles have V=0 at the end. However, we'd all agree the one using his brakes suffers less! The question is the time interval over which the momentum is lost. If it's instantaneous, the energy applied is infinite, much to the chagrin of the driver! So, if the wall isn't very strong (it's very flexible) the time interval over which the momentum is lost could be appreciable compared to the two vehicles colliding, and the energy applied will be less than the two cars colliding!

Wrong on Audi A4

by WoomRR

I believe the A4 driver who heard wow-wow-wow with increasing frequency when turning left was hearing the tires. He told you that during the alignment the mechanic said his tires were worn. You suggested it might be power steering.

Favorite Moment: Lillian and Bobby. That call made be pay attention to the A4 caller.


You know it's time for a new car when...

Yeah, these are unmistakeable.

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