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Re: help with air friction?



I have good success with foam balls (3.5 cm radius, about 3 grams in mass)
that you can buy from Oriental Trading for about $3/dozen. These reach
terminal velocity after only a couple meters fall. We time a variety of
falls of the range 20 cm to over 10 meters (our library provides the long
drops). The short drops are timed (with some difficulty) with photo-gates,
while everything over about 1 meter can be hand timed.

I then have students model the process using spreadsheets with the drag
coefficient calculated from .5rhoCA: rho = density of the medium, C the
geometric factor (.5 for a sphere) and A the cross sectional area. Yes,
this is cheating somewhat since this IS the factor for v^2 dependence, but I
have them model the air resistance force as a v^n dependence and have them
start with n = 1. Their data clearly doesn't fit this (so small corrections
in 'b' aren't critical--the units from above don't really work). For n = 2
they get really very good fits. We don't consider buoyancy but then the
data are not that precise.

Rick
-----Original Message-----
From: LUDWIK KOWALSKI <KOWALSKIL@alpha.montclair.edu>
To: phys-l@atlantis.uwf.edu <phys-l@atlantis.uwf.edu>
Date: Wednesday, November 19, 1997 11:21 PM
Subject: Re: help with air friction?


If I was to advise Tom Fredrick who wants to find how the air resistance
force depends on v I would suggest measuring three or more v_terminal for
spheres of different masses (same radius). The air resistance is m*g for
each sphere. Free vertical fall, naturally. Boyancy correction for very
precise data. Easier to say that to do.