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[Phys-L] Re: coefficient of friction or drag



Jeremy Miller wrote:
I am planning to conduct a model rocket project with a high school physics
class and I wanted them to collect data on their rockets.
One of the variables that I wanted them to determine is the coefficient of
friction or drag on the parachute as the rocket came back down.
I wanted to get an opinion on the best way to accomplish this.
My first thought was to have them find the velocity of the rocket on its
descent and calculate the velocity of the rocket where there is no friction.
Then use the frictionless rocket as the accepted value and the velocity they
found for their descent as the experimental value.
(Accepted value minus experimental value)/ accepted value. The difference
in the velocities is due to friction or drag.

As Karl pointed out, the definition of drag is not "the difference in
velocities". This is a terminal-velocity situation. You don't even
need to know the properties of the hypothetical frictionless rocket.

What might be a better way?
They will know the beginning mass, ending mass, altitude, and time of flight
both up and down.

Yes, there is a better way.

A real rocket scientist would use a wind tunnel. Actually, the 'tunnel'
strictly speaking is optional; the key elements are
-- automobile, for generating your own wind
-- anemometer, to measure the true airspeed (i.e. your groundspeed
plus nature's wind)
-- jig to attach the model to the car
-- strain gauges on the jig to measure the forces.

If you think that's a lot of work, bear in mind that measuring the
height-versus-time of rocket in flight is a lot of work, too. I can
imagine doing it by capturing the flight on video (with at least two
cameras) and then analyzing the recordings, using lots of trigonometry,
but the wind 'tunnel' is almost certainly easier and more accurate.

Making the jig is a bit of an investment, but you get to amortize
it over multiple years.