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#0821: No Poorly Done Deed Goes Unpunished

Original Air Date: 05.25.2008

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Description: 
This week on Car Talk, good intentions go awry for Drew in New Jersey. He decided to surprise his girlfriend Sarah by washing her car, and--oops!-- left a very large and messy scratch. She's kinda ticked off, and her mood probably won't improve when she realizes his restitution will require her wallet. Meanwhile, out in Colorado, another family is in need of Magliozzi Mediation. This time, it's Tyler and his dad, who can't agree on what kind of car crash causes the most damage. We think they might want to lighten up just a little, but Tom and Ray nonetheless take a step back to their year(s) of Physics 101 to try to help. Also, a mysterious backfire makes one woman think her husband may be trying to knock her off, a skier wonders why he's become a member of the Brake Pad of the Month Club, and what do you do when a mechanic tells you your carburetor needs work... when you don't have a carburetor? All this and more, on this week's Car Talk.

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This Week's Puzzler

Most cars have one, some have two, none have three and some have four. What is it? Find out!

Last Week's Puzzler

How do you plant four plants... and keep them all equidistant? Find out!

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Could Infiniti's Plasma Cluster Filtration be the solution Tommy's been looking for all these years?

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Show Review - 411

by Anonymous

Two car collision: your answer was erroneous. Think of it this way: if you put a paper-thin wall at the exact point of collision of two cars going 60 mph each, each driver would be under the impression that she is colliding with a wall. So, the head-on collision at 60mph each is the same as a collision with the wall at 60mph. Don't try this at home.

Show Review - 415

by Anonymous

Show Review - 416

by Anonymous

I agree with the other reviewers that you guys got it wrong on the 24 May show in the phone call about whether a collision of two cars colliding at 60 mph each is the same as one car colliding with a rigid wall at 60 mph or 120 mph. You said it is like hitting a wall at 120 mph. It is closer to colliding with the wall at 60 mph. Here's how I would explain it: I think we're all assuming that the two cars have identical mass and roughly the same height - two cars of the same model and weight. If one is a Mack truck and the other a Preus, then it's a lot tougher on the Preus than the truck. But we're assuming they're pretty much identical. You remember from physics (right?) that the reference frame doesn't matter to the physical outcome of an event. Suppose then the reference frame were moving with one car. Then, in that reference frame, that car is not moving, but still, and the other car approaching at 120 mph. Sounds like a car approaching a wall at 120 mph, right? WRONG. After the collision, when both cars are at rest, in this reference frame they'd both be moving backwards at 60 mph. The collision isn't like hitting a rigid wall at 120 mph, it's like hitting a car sitting at rest (in neutral, without brake on) at 120 mph, and then with both cars rolling along at 60 mph after the collision. Look at it another way. Each individual car experiences a sudden decrease in its velocity of 60 mph at the moment of the collision. That is like hitting a rigid wall at 60 mph. The car and its occupants do not experience a sudden change of velocity of 120 mph, as would occur if a car hits a wall at 120 mph. In one sense the collision IS worse than hitting a wall at 120 mph. The analysis above assumes that each item in each car is perfectly matched, so that each component of each car hits its opposite number at the plane of the collision and comes to rest. If the other driver has solid-lead dashboard decoration lying unattached on his dashboard, it would come crashing through your windshield at 60 mph, as you come to a sudden stop. That would be a difference of 120 mph in velocities. But for the car as a whole, it's like hitting an immovable wall at 60 mph, not 120.

Show Review - 410

by Anonymous

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Boy, did you blow it!

by larryj

our response about the two cars approaching at 60 mph each was dead wrong. Try this: you're going 60 mph and can crash into a concrete wall or another (stationary) car. Which would you choose? If this doesn't lead you to the correct answer to the oriignal question, I'll explain further, but you have found a way to violate the law of conservation of energy - a very useful ability in this day of high prices for fuel.

Favorite Moment: I tried to call in to save you some long-term embarassment, but the line was busy - I figured a couple dozen other chemists (and a few physicists and maybe an engineer or two) were trying to get through to you. Hope I did with this message.

car crash physics; 60=120???

by breenrichardj

Sorry, but for once in your career you were WRONG! Two cars hitting head on at 60 mph (each) is NOT the same as one hitting a wall at 120. Basic Physics: Force is Mass times acceleration F=MA Energy is 1/2 Mass times velocity times velocity; E = 1/2 m Vx V ( v squared) Force brings the car to a stop, Energy causes physical change, bent broken metal ( bloody people) Assume the car has mass 1, and 60 for speed case one head to head, EACH car has Force (momentum) of 60. and 1/2x60x60 of energy. Each car comes to a stop due to force on the bumper and is crushed by 1,800 of energy. case two car into wall. Force of 1x120 but energy of 1/2 x120 x 120 that is 7,200 crash energy. PS I,m not making this up, I have a BS in Physics and experience as aerospace engineer. Don't feel bad most people in this country are poorly educated in math and physics.

About two cars.

by Mike_L

It's amazing, how wrong you are.This is not your fault.It is AMERICAN high education.MIT, ha?

Mistake on Crash

by John Hilgers

A car going 60 mph crashing into a wall is the same as a head on crash with another car of the same mass also going 60 mph. It's a simple problem of inelastic collisions and conservation of momentum. See: "Head On Crash" at www.exo.net/~pauld/books/car_science/miscellaneous_car_science.html Love your show.

Newton's Law

by CarTalkListener

Newton’s Law: For every action, there is an equal and opposite reaction. When you hit the oncoming car, that car exerts a force on your car equal to and opposite of the force of your car’s impact. When you hit a solid wall, that wall exerts a force on your car equal to and opposite of the force of your car’s impact. The forces are the same. Love your show.

A Wall of Marshmallows?

by jmkray

The 2 cars hitting each other head on vs the solid wall answer was off, I think. The solid was (of brick,etc.) would have no energy absorbing qualities, but an identical weight/model/hitting an identical weight/model would dissipate energy more than a solid wall (if the wall were made of jello, no problem!). The moral issue of what about the poor slob(s) in the other car bring another factor into the equation...I would go for the wall, but I'm not you too guys! That's joke!

Favorite Moment: Every show all the time. I don't care if PBS wants to yank ya. Tell Dot Matrix, "Hi!".

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