In this experiment you are to study a forced, damped harmonic oscillator using a linear air track. The driving force is a variable speed motor geared down to a shaft with an off-center driving point. The oscillating body is a "car" on the air track and the damping is obtained by means of a metal vane on the car which passes through a magnetic field. As the vane moves between the poles of the magnet, eddy currents are set up in the vane which interact with the magnetic field to create a retarding force. The force is proportional to the velocity of the car. A schematic of the setup is shown below.
The magnet and the metal vane on the car must be carefully aligned so that the vane passes between the poles of the magnet without touching. The springs used in this experiment are extension springs, that is, they must be kept under tension in order to work properly. If they go slack the spring constant changes abruptly and the so do the conditions of the motion. Hence the displacement amplitude of the car must be maintained below the point where the springs or the driving string goes slack during any part of the cycle. The amplitude of the driving force is adjustable. You will see that it is necessary to keep it small to avoid violent oscillations near resonance.
Before describing the experiment in detail, a few DO's and DON'Ts regarding the air track must be mentioned. To begin with, please realize that no one has had much experience with the condition of near-zero friction (unless you've already done the Collisions experiment). As a result, there is a tendency to supply too much energy to the cars on the air track. So the first rule is to work with small forces.
Some DON'TS:
lean on the track - it bends.
rub your fingers across the holes.
Minute amounts of oil from your hands mixed with dust will
clog the holes.
mark the track with pencil or pen.
allow the cars to bang against the track
as small dents will cause irregularities in the motion.
allow the cars to scrape against the track
during their motion.
This wears out the track as well as introducing unwanted friction.
leave the air supply on when it is not being used.
You MAY:
slide the car slowly on the track when the
air is off for final
positioning. It is better to slide it a little way than to pick
it up and risk banging it down again. However, don't slide a
car the whole length of the track.
use tape to place markers on the track.