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Re: MBL, was Re: Air resistance




-----Original Message-----
From: John Gastineau <gastineau@mindspring.com>

One experiment I try every
year is using a tilted air track to study accelerated motion and in part
to
measure the acceleration due to gravity. This never works very well (the
technique requires a photo-gate to be triggered AS SOON AS the cart starts
to move down the incline and that is difficult to set up properly).

If the technique has an obvious flaw in requiring a difficult-to-achieve
setup, why use it? There are many other ways to measure g using a
photogate, and they work very well.


Well I continue to use this because IT IS an air track experiment where
we're looking at basic definitions of motion and basic graphing skills.
Half of the experiment is to launch the carts on level tracks measuring
distance as a function of time, graphing, and finding the speed (velocity)
from the slope. Using a tilted track with the first photogate right in
front of the cart WILL show that this motion produces a non-linear distance
versus time graph. However, using the assumption (with this geometry) that
the initial velocity is zero and that the acceleration is constant, the
instantaneous velocity at the second photo-gate is just twice the average
velocity and a velocity versus time graph IS linear and an acceleration can
be obtained. Knowing the angle of the track--obtained from knowing the
height of the raising block and the distance between the feet of the air
track, one can use the experimentally determined acceleration to calculate
'g'. Agreement is normally +/- 20% which is OK with me but disconcerting to
many beginning students. Later we measure 'g' with a pendulum to much
greater accuracy (but only as a side-bar to studying the motion of the
pendulum itself).

Rick