Hacksaw blade resonator

Class experiment

Something like a blade can oscillate on its own at a definite frequency. Here students investigate what happens when it is driven by an oscillating force at a rate which may or may not be the same as the natural blade frequency.

Apparatus and Materials

• Hacksaw blade oscillator
• Hacksaw blade
• Meccano strips (No. 1 and No. 2a) (if not with item 1024)
• Stopwatch or stopclock
• G-clamp (large)
The following items should also be available, if required:
• Metre rule
• Postcard, cork, needle, rubber band
• Retort stand base, rod, boss, and clamp 2

Health & Safety and Technical Notes

Assemble the oscillator as shown in the diagram. You may need to substitute Meccano parts and the hacksaw clamping device with something similar.

The amplitude of the motion of the driver pendulum can be maintained by gently tapping the pendulum strip a little below its support with one finger.

Read our standard health & safety guidance

Procedure

1. Investigate factors affecting this oscillating system, consisting of a hacksaw blade driven by the heavy pendulum.
2. Before coupling the blade to the 1 kg pendulum, find out how to change the natural frequency of each of them.
3. Try changing the degree of coupling in the system, by using different rubber bands (or springs).
4. Damping may be changed by turning the postcard so that it is at right angles to the direction of motion of the blade.

Teaching Notes

• Students could be given a very loose brief, to investigate how this system behaves. An open brief can stimulate students to think for themselves and to closely observe what happens in this oscillating system.
• Alternatively, students could be divided into pairs or small groups, with each one investigating a specific factor, for example how the system responds with standard coupling, with different degrees of coupling or with damping. In each of these cases, they might collect data showing how the amplitude of hacksaw blade oscillations varies across a range of driving frequencies, concluding by drawing a resonance curve.
• Whatever approach you use, set up one sample apparatus beforehand so that students do not waste time trying to figure out how to set the apparatus up.  You may also want to suggest that they observe not only blade oscillation amplitudes but also any phase difference between the blade and pendulum oscillations.

• The natural frequency of the hacksaw on its own can be found by displacing it from its equilibrium position and releasing it to oscillate freely. The frequency is determined by counting, say, the time for 10 complete oscillations. Students should find that the natural frequency of the hacksaw blade depends on its length. A similar procedure can be used to find the natural frequency for the 1 kg mass, which will depend on the length from pivot to the centre of the 1kg mass.
• These investigations provide a good opportunity to practice what may be new vocabulary, using terms such as ‘frequency’, ‘amplitude’, ‘phase’ and ‘resonance’.
• Adequate time is essential. A single long practical session may not be enough.

This experiment has yet to undergo a health and safety check.