Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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



At 03:56 PM 2/24/2007, Ralph vP, you 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

I heard it from my son John, who heard it from....
...but don't tell anyone!

1) Take all possible weight out of the wood frame.
2) Add lead at the bottom rear to the upper limit allowed.
3) ensure that the front axle is canted so that only
one front wheel bears weight.
4) optimize the body shape for low air drag.

You will easily see the virtue of this recipe when
I mention the rationale:
1) & 2) The pinewood derby cars are launched from a ramp.
Maximizing the potential energy (with the tail weight high)
is naturally optimal.
3) & 4) Air drag is reduced by good shape - wheel friction is
reduced by keeping one wheel hors de combat.

Don't tell anybody now!


Brian Whatcott Altus OK Eureka!