Introduction to oscillations
Practical Activity for 14-16
A circus of experiments involving repetitive events, to introduce the concept of oscillation.
Apparatus and Materials
- Oscilloscope with a slow time base
Station B: A rubber ball bouncing
- rubber ball
Station C: A scaler counting regular pulses
- signal generator
- 4mm leads, 2
Station D: A scaler counting slow, random counts from a GM tube
- GM tube holder
- thin window GM tube
Station E: A rotating turntable
- motor-driven turntable
- card, adhesive tape
Station F: A watch or clock ticking
- stopwatch or stopclock
Station G: A slow-running flashing neon circuit
- h.t. power supply
- resistance substitution box, 1 MW
- capacitor, 1 mF 500 V
- clip component holder 2
- neon lamp and m.e.s. holder
- 4 mm leads
Station H: Dripping water
- burette, stand, and beaker
Station I: Bar magnet suspended over another magnet
- cylindrical magnet
- horseshoe magnet
- retort stand base, rod, boss, and clamp
- nylon fishing line
Station J: Large-amplitude pendulum
- turntable clamped vertically (or large gyroscope)
- boss (or small G-clamp)
- retort stand base, rod, and boss
Station K: Air track vehicle running between elastic barriers
- air track with rubber band barriers at both ends, and blower
- air track vehicle
Health & Safety and Technical Notes
Set the time base at 100 ms cm-1 and the stability control at maximum.
Station B: The ball must bounce gently on a hard surface.
Station C: Set the signal generator to give square waves at about 100 Hz. Connect the high impedance output terminals to the unpolarised scaler input. Increase the output voltage slowly until the scaler counts regularly.
Set up the apparatus to show the slow, random background counts from the GM tube.
Tape a short piece of wire to the turntable and fix the card so that the wire just touches it as the turntable rotates.
If necessary, set up a microphone, amplifier and loudspeaker so that the clock ticking is audible.
Connect as shown below.
Arrange the burette flow rate so that water drips slowly from it and is roughly steady over short times, but decreases over long times.
Hang the bar magnet on nylon line so that it’s horizontal and lies just over the poles of the horseshoe magnet which rests on its back. NOTE: If you attach a small piece of mirror to the suspension, students could also observe the oscillations by optical means.
Clamp the turntable with its axis in a horizontal orientation, so that the turntable turns in a vertical plane. Clamp the boss (or G-clamp) onto the edge of the turntable so that the system can be made to execute large-amplitude oscillations about the lowest position of the boss.
Set up elastic band barriers across the air track, about 0.5 m apart, so that the vehicle rebounds between them with little loss of energy.
- Carefully observe the motion at each station. In some cases it will be important to time the oscillations. Afterwards, in pairs, discuss the following questions.
- How could you tell if an event repeated regularly? Use a clock? How would you know that the clock ticked off equal time intervals? What would be observed if pairs of these repetitive events were compared with each other for rate and for regularity? Which of these events count as clocks? Which are good clocks?
- Set up a suitable circus of experiments, selecting from those listed, so that the class can consider a good range of repetitive events. Make sure to include a few that do not repeat regularly (stations B, D, H, I, J, K). You might also include
- When appropriate, facilitate a whole class discussion, basing it on the pair discussions which have taken place. In general, oscillation involves going back and forth repeatedly between two positions or states.
- Which stations (systems) produce isochronous oscillations (good timekeepers)? Students might use clocks at home to test their own pulse rates for regularity. (Galileo used his pulse to test a pendulum, or so the story goes.) Ask whether irregularity matters - is it possible to use radioactive decay as a clock? (Some students may have heard of radiocarbon dating.) You might also refer to modern time standards and to astronomical methods of time measuring. And you might also ask more philosophical questions, such as "Could time run backwards?" and "Would we know if it was doing so?" The direction of time for an elastic collisions, for example, is indistinguishable, but an arrow of time is suggested by the concept of entropy and the 2nd law of thermodynamics.
This experiment has yet to undergo a health and safety check.