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#1304: How to Lose a Pinewood Derby

Jan 26, 2013
This week on Car Talk, Kathy is "helping" her son build his Pinewood Derby race car, and is looking for a few design tips. Can two MIT grads-- one of whom failed Physics twice-- lead her to Victory Row, or is this Cub Scout Den Mother barking up the wrong radio show? Elsewhere, Sophie thinks her Saab is laughing at her, and not just because it's delighted by her French accent; Ainsley's husband won't let her take her Honda to the car wash because he's worried it'll melt the Epoxy that's holding the car together; and Richard won Employee of the Month by driving 95 mph to deliver his boss' passport to the airport. But, will he be forced to sell his plaque to fix the problems that have plagued his Jeep ever since? And, our Historic, Folkloric Question of the Week comes from Julie: why do we drive on the right, if we're a former British colony? All this, and lots more, this week on Car Talk.

Show Open Topic

Does pushing the elevator button more than once make it arrive faster, and other questions to ponder.

This Week's Puzzler

What product did the company sell that it could not demonstrate in its building?

Last Week's Puzzler

A thousand, one-dollar bills go into ten envelopes. How do you distribute them to always make correct change?

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9 Comments

Pinewood

Has anyone actually done controlled experiments to test all these theories on Pinewood Cars… I mean, other than by the Cub Scouts (too many uncontrolled variables)? By the physics formulae it is simple… all weights accelerate by gravity the same… but more weight carries more momentum down the track (opposing the losses due to friction and drag). But the clunkiest cars slapped together at the last minute, full of Lego drivers and wild fins etc (dreamed up by some 8 year old kids) always seem to win over the most slickly engineered rods created by "supportive" dads. So how come?
Favorite Moment: 
This mom takes momhood too seriously, but is enjoying her time in cub scouts!
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Of course the car needs to be heavy

More mass means more momentum. The friction (air drag is negligible) applies a constant force. Constant force takes longer to stop a high momentum car than a low momentum car. The formula you were looking for is a=f/m. Note that deceleration is smaller when mass increases.
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I like the whole thing
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Pwd correct answer

Potential energy at top is mgh, thats all you get, directly proportional to m with a factor of 1.0. Everything else is about minimizing the losses. Some of those losses are also proportional to m, but by a friction coeff. (Typ less than 0.1). Therefore maximize weight.
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Pinewood Derby - BOGUS Answer

Review.Review. Car can weigh no more than 5 ounces. All cars travel the same distance down the 45-degree slope, and one car goes further than any of the others along the flat. Although the front of the cars all travel the same distance, where you place the dense weight makes a big difference. If the weight is at the front, the acceleration ceases as soon as the front levels out. If the weight is at the rear, then the weight travels a greater distance before the weight makes the transition from the slope to the level. That 6-inch car length gives the weight at the rear of the car an additional 4 1/4 inches of vertical drop, resulting in a higher terminal velocity. In fact, until the car is 6 inches along the horizontal track, the weight in the rear is still accelerating.
Favorite Moment: 
It has absolutely nothing to do with cars, or driving, or.............
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The derby

potential energy of the car is Pe=m*h*g where m is mass of the car,h is height from top to bottom of the stand, g is acceleration due to gravity. Kinetic energy is Ke=1/2 *M*V*V where v is velocity. at top of the ramp Pe is max, Ke is 0, at the bottom of the ramp Ke is max, Pe is 0. Drag energy we can make De=Fd*D where d is distance, Fd is drag force. SO Pe turns into Ke and that turns into De. the limiting factor is Fd, which is force of drag, comes from wheel drag, air drag, etc... Just by looking at this, the car must be made heaviest so it will have maximum Pe which will yeld maximum Ke. As long as Fd is small, then D will be largest.
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Pinewood Derby (PWD) Knuckleheads!

You both deserve D'oh-p Slaps! Any experienced PWD parents KNOWS the weight goes in the back! May I refer you to the following lectures by Dr. John Jobe, who didn't graduate MIT, but DID pass his physics class. http://www.pinewoodderbyphysics.com/pdf%20files/Lecture%207.pdf http://www.pinewoodderbyphysics.com/index.shtml
Favorite Moment: 
Pinewood Derby call from Kathy. Although Tom & Ray completely blew the call!
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pine wood derby

The weight on the car should near the back axle. The physics have already discussed Momentum=mass x velocity. Many factors effect velocity (friction, track conditions, etc.) Since most cars start at the same speed, then the only control you have is to control the mass.
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idea

1. re Tomiko from MI; wonder if sunflower seeds are not from plant, but some neighbor has 50lb bag of sunflowers in garage which rodent accesses and stores in Tomiko's car. 2. on a show some weeks ago, woman called in about Honda burning oil ever since she moved, but also she had over 100k miles on the car. I know the answer to this because I have been driving Hondas since 1975 (first one - little blue civic (called "snow car") purchased for $2883). I drive every car to over 200,000 miles. It is my experience that with every single Honda I have driven, once over 100,000 miles, they burn oil. This is my consistent finding and that is all there is to it. It is the Honda MO. I am always surprised mechanics do not know this, but realize unless they have driven as many Hondas and for as long as I have, they might not know this fact. Thanks
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Pinewood racing - it's serious

The caller wanted to know if her son should be adding weight to the rule maximum. YES! This is a race where all the energy the car has at the start of the race is the potential energy (mass*height*gravitational acceleration constant). Since all cars start at the same height, if all else is equal (primarily wheel friction and losses due to not running straight), then the one with the highest potential energy wins. Don't focus on appearance, works the axels and wheels. Good luck to all Cub Scouts this spring as you race in the Pinewood Derby.
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