measuring drag on a soap box derby racer?

[peteryoung]

This is an interesting question, dynama_girl. I think measuring drag
directly would be a useful objective as distance travelled is very sensitive
to wheel axle alignment, friction, etc.

Here's a suggestion: you can get an inexpensive postal scale at any
stationery store - usually scaled to measure just a few ounces in
fairly fine increments. A spring type scale should not cost much or perhaps
you can find one at a yard sale.
Set up your racer in your 'low tech' wind tunnel so that the racer
transfers its drag force straight back to the postal scale - using a dowel or rod to transmit the force. You'll probably want to remove the piece of
plastic from the scale that usually holds the envelopes being
weighed - it'd contribute undesirable drag on the measurements.
Run a series of runs at various fan speeds and record the results
carefully. Do you have ways to estimate or measure airspeed in
the wind tunnel? There's an fairly simple (and inexpensive)
wind meter used by outdoor sports people - basically a light
plastic ball that rises up and down in a larger cylinder as a function
of airspeed.

Peter Young

[ChrisG]

Just for clarification, this post is a response to this thread:
http://www.sciencebuddies.com/mentoring ... php?t=2989