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Re: [Phys-l] Pinewood Derby Car Weight....



At 14:49 -0500 2/25/07, Patricia Sievert wrote:

We have years of experience with pine wood derby competitions in our
family. The PE to KE explanation already given is what we used to help
our sons do quite well. We never spent much time on balance and
stability or they may have done even better. If the starting gate holds
the cars at an angle (can't picture it any other way), even placing more
of the weight towards the back of the car can make a difference.

Mentioning the extreme case, our daughter is younger than both our boys
and one year we made a car for her that was WAY over the 5 oz. weight
limit. The night before the official races our Scout Pack had trial
runs. The only rule being that you couldn't test your car against
someone in your same category. Sara had a blast racing against all the
little boys as her plain boxy-looking car won every time, and not just
by a little bit. It wasn't until she let one of her friends hold her
car that they caught on.

It is certainly true that the source of energy to move the cars is the conversion of potential energy.

But the issue is not that putting the mass at the back will make that much difference (as was pointed out, since the cars all end up level, although they started on a slant, it does make a bit of difference). The gain is in the increased mass that reduces the effectiveness of air resistance.

If you eliminate all friction from the calculation, we all know that the final velocity of an object after it has fallen through a given difference in height will be independent of the mass. Moving the mass from the middle of the car to one end cannot make a difference of more than a centimeter or so in the height difference between start and finish, and that won't help much (although it might make the difference in a close race).

But the effect on the speed just due to increased mass can be significant in reducing the effectiveness of the air resistance. I showed in my earlier post that the contribution of the mass to the effect of air resistance on the acceleration is inversely proportional to the mass, so doubling the mass reduces the effect of air resistance by half. That is very important, especially if the design of the cars is not aerodynamically optimum, which, in these situations is unlikely to be the case.

Hugh
--

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Hugh Haskell
<mailto:haskell@ncssm.edu>
<mailto:hhaskell@mindspring.com>

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Hard work often pays off after time. But Laziness always pays off now.

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