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

I suspect that the posters who don't follow the PE argument may not be
familiar with the setup for pinewood car derbies.

Imagine a shot ramp that launches a car onto a level surface. I think
everyone would agree that the resulting speed of the car depends on
whether the weight is distributed more over the rear wheels than the
front wheels because of the position of the center of gravity and its
resulting displacement.

The same occurs in the derbies when a short ramp launches the cars onto
the actual racing ramp.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Patricia Sievert
Sent: Sunday, February 25, 2007 2:50 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Pinewood Derby Car Weight....

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.
Pati

von Philp wrote:

My young son was recently involved in a pinewood derby car race at
our
church. Since I have never been involved with pinewood derby cars in
any
way, I tried getting some information from the internet. Most of the
information I found strongly suggested making the car as heavy as
possible.
One site specifically says "Don't even think about skipping weight
addition
if you want to be race competitive." and "The weight of your car
overcoming
friction is what will allow to you to win over other cars. You must
make
gravity work for you."
Now, I really don't want to argue about the fact that heavier cars go
faster, since the people that wrote those statements probably have a
ton
of
experience with this sort of thing and I think they would know. But
I'm
not
sure I fully understand why the greater weight gives the cars an
advantage.
One site says the greater weight gives it more energy and therefore
greater
speed at the bottom, but this is an incorrect reason due to energy
conservation being independent of mass. Another site recommends
putting
more weight toward the back because it puts more weight higher giving
it
more (potential) energy. Again, while these people may know their
racing,
they don't know their physics.
A finished car will typically weigh around 70 grams or so. The
maximum
limit is around 142 grams (5.0 ounces). They recommend adding weight
to
bring it up to as close to the maximum as possible. Could those
extra 70
grams really make much of a difference (all else being equal!), or
could
they just be speculating without any supporting data. It would be
nice
to
be able to test the same car and add different weight amount to see
if
the
race time is affected.
I'd like to know what the rest of you think about this.
~ Ralph von Philp

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--
Patricia Sievert
Physics Outreach Coordinator
Department of Physics
Northern Illinois University
DeKalb, IL 60115
sievert@physics.niu.edu <mailto:sievert@physics.niu.edu>
(815) 753-6418
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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