Dear Tom and Ray;
I'm throwing this letter into the pile of what will no doubt be a veritable tempest. This storm of mail will revolve around your attempts to help a woman design a Pinewood Derby car. I believe it was Tom who maintained that the mass of the vehicle didn't matter, even after spouting off almost all the equations and concepts one would need to quantitatively demonstrate that this is not the case.
Allow me to explain. You stated that two objects of different
masses falling under the influence of gravity will fall at the same
rate. This is true only if you neglect the effects of friction. If
you account for friction, however, then heavier objects will indeed
fall faster! Try this experiment. Take a feather and a large hammer
(the bigger, the better). Take off your shoes. Now hold the hammer
over your right foot and the feather over your left foot. Better yet,
have your brother do it from a stepladder. Now drop both at the same
time. Won't that light touch of the feather feel nice?
The first third of the track is quite steep and the cars are relatively close in mass, so we shouldn't be surprised that they reach the bottom of the portion at roughly the same time. It is in the long runout that follows that friction becomes truly significant. This is where F=MA becomes important. The force is the frictional force working to slow the car down. There is only so much you can do with wood, nails and pencil lead. The parents (oops, I mean kids) who put moderate effort into drag reduction will all have roughly the same amount of friction to contend with. The F in the above equation will be constant, so if you double the mass, the deceleration due to friction will be cut it half.
Of course I have oversimplified things for the sake of brevity, but I'm sure you get the picture.
Yours truly,
Peter Howell
Analytical Chemistry Graduate Student
Florida State University
