I was asked a similar question, based on observations of the relative speeds of downhill skiers of differing mass. I had to ponder for a long time before coming up with this explanation:
The force of air resistance on two skiers should be about the same if they present the same profile. The *effect* of that force--acceleration--will be affected by the mass of the skier; the less-massive individual will experience a greater (negative) acceleration.
--Daniel Price, Columbine High School
-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of von Philp
Sent: Sat 2/24/2007 2:56 PM
To: Forum for Physics Educators
Subject: [Phys-l] Pinewood Derby Car Weight....
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