Re: timing data for falling objects

["John S. Denker" <jsd@MONMOUTH.COM>]

Robert Cohen wrote:
...
> I just want some "real" data to illustrate that
> ignoring air resistance is okay for short
> distances.

Just be careful what you mean by "short" distances.
The relevant length-scale for dropping a party-balloon
is very different than for dropping a bowling-ball.

If you're not careful you could wind up "illustrating"
something that isn't true.

> I don't want to make up or simulate data.

"make up" seems like unnecessarily pejorative language.
There's a difference between "made up" fictional
fairy-tale data versus well-founded calculated data.
You can easily calculate the drag well enough to satisfy
the stated purpose.

===============

Constructive suggestion:  Make a spherical pendulum.
For example, dangle a baseball on a string.  Set it
in circular motion as accurately as you can.  Observe
the timing and the size of the orbit as it decays.
Observing this for a few seconds is incomparably easier
than observing a falling object for a few seconds.
 From this data it is straightforward to determine the
magnitude of the drag force, as a function of velocity
over a wide range of velocities.  Repeat with golf ball
and ping-pong ball.

This isn't as accurate as a wind tunnel, but the results
are easier to interpret.  (A wind tunnel is easy to make,
and fun, and useful -- but it's hard to calibrate the
wind-speed in the tunnel in a way that high-schoolers
find easy to interpret.  A spinning-cup anemometer is
pretty mysterious, and so is a Pitot tube.)

===============

Also keep in mind that a ball is a notoriously non-
streamlined shape, so you can expect results to be
far from perfectly reproducible.  This is good;
students ought to be exposed to the fact that in
the real world there are ill-conditioned and chaotic
systems.  Knuckle-ball aerodynamics is an example.