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



Let's (over)simplify things a bit.

They have an altitude (perhaps approximated from the specific impulse
of the engine as well as a triangulated value?)

They have the time of descent (stopwatched from seeing the parachute deploy?)

If the parachute is 'reasonably' large - terminal velocity will be
reached quickly.
so we can calculate this terminal velocity


Draw the Force Diagram for the terminal velocity phase.
Gravity force down = drag force up

Use different sized parachutes to check whether the drag is actually
proportional to the area.

Use different final masses (to get different final velocities) and
check that the drag force depends upon velocity raised to some (to be
determined) power.
Use a power-law fit of drag force vs velocity to approximate this exponent.
(This data/result will not be 'pretty' - but it's real!)

Yes - it's oversimplified - but it covers a lot of stuff that they
know AND helps you justify buying all of those Estes rockets and
having as bunch of FUN!!!!!

Have them read October Skies (Rocket Boys) or at least watch the flick.


At 2:13 PM +0000 12/23/04, 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.

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