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

["John Clement" <clement@hal-pc.org>]

Of course the idea the friction is proportional to the normal force is only
approximately true.  But beyond that the air resistance is not affected by
the normal force so if you can increase the gravitational force while
decreasing the air resistance, the answer is obvious.  So essentially they
did not know that friction increases with normal force, and they did not
consider air resistance as a separate effect.  There is also the possibility
of increasing stability by lowering the center of mass.

John M. Clement
Houston, TX


> > 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. ...
>
> Ralph,
>
> One mental exercise that may help you appreciate the role of extra
> weight--and, more importantly, the physical mechanism that gives it
> an advantage--is to imagine the results for the opposite extreme--a
> very light car, say ten grams.  Make it one gram, or a tenth of a
> gram, if necessary.