One version of what John refers to is a 'shaft encoder'. We use these for pendulum
experiments, and many other applications.
They can be mounted as pulleys at the end of the track. A light cord over the
pulley gives the displacement of the cart vs. time. The resolution of the encoders
is outstanding - much better than sonic rangers.
An especially nice experiment is to couple two air carts with springs and have a
thread from the cart nearest the end over the encoder pulley to a light mass. It's
then quite easy to obtain the two mode frequencies of the carts.
We have used a shaft encoder coupled to a light detector so that when you push the
detector through a diffraction pattern, you immediately get a plot of intensity vs.
distance.
Encoders are also used coupled with magnetic field detectors and an air track to
plot the axial magnetic field of a coil vs. distance.
Mike
--
Mike Moloney, Physics & Applied Optics Department
Rose-Hulman Inst of Tech, 812 877 8302
moloney@rose-hulman.edu http://www.rose-hulman.edu/~moloney