# Period and frequency

##### Class practical

This is a useful activity for dealing with the concept of frequency.

#### Apparatus and materials

For each student group

Hacksaw blade or similar metal strip

Metal strips used as jaws, 2

Hand stroboscopes, 2

G-clamps, 2, 10cm and 5cm

Stopwatch or stopclock

#### Health & Safety and Technical notes

Although hacksaw blades are traditionally used for this activity, some schools may consider it necessary to use strips of hard steel without teeth.

#### Procedure

a Clamp the hacksaw blade (or similar metal strip) tightly between the two small metal strips using the larger G-clamp. Clamp the whole set-up to the edge of the bench.

b Set the blade vibrating and see if you can count the oscillations.

c Slow the oscillations down by fixing another G-clamp to the outer end of the blade. (You can wind a rubber band onto the handle of this clamp to stop it from rattling.)

d Measure the time for ten complete oscillations and work out the time for one oscillation.

e Work out how many oscillations take place in each second.

f Move the clamp inward. This increases the frequency of oscillation. Use a hand stroboscope to 'freeze' the motion. Discuss how this works. The stroboscope should be held so that the observation slit is parallel with the vibrator length. Only one slit should be open.

#### Teaching notes

1 A complete oscillation involves returning to the same position, travelling in the same direction. The time for a complete oscillation is called the ‘periodic time’ or  ‘period’, symbol T. The number of oscillations each second is called the ‘frequency’, symbol f and unit hertz. One hertz means 1 oscillation per second.

2 It is important to clamp the blade tightly and symmetrically just where it emerges from the jaws. Otherwise energy is lost rapidly and the oscillations are badly damped.

3 A challenge for more advanced students: provide a ticker-timer, power supply and tape and ask them to solve the problem of determining its frequency.

4 The stroboscope should be rotated, gradually faster and faster, until the maximum speed at which the blade appears 'frozen' is reached. (If two or more blades appear frozen then the speed of the stroboscope is too high.)

5 With faster vibrations then two stroboscope slits need to be opened diametrically opposite. Two glimpses will be seen with every rotation of the stroboscope, so the time for a single oscillation of the blade will be half the time for the rotation of the stroboscope.

This experiment was safety-checked in March 2005

#### Related guidance

Oscillations and clocks

Oscillations, waves and mathematical models