Harmonic Knives

This demonstration shows how a knife vibrates when struck and how holding it at different points changes the sound produced. By gripping the knife at a node, vibrations continue and produce a clear tone, while holding at an antinode dampens vibrations and creates a dull sound.

1. Hold a table knife gently by the handle and strike the blade with a pen. Listen to the clear ringing sound.
2. Hold the knife at the base of the blade and strike it again. Notice the change in sound.
3. Try holding the knife at the end of the blade, then at different points along the blade and handle, striking it each time.
4. Compare the sounds produced and identify when the knife vibrates freely versus when vibrations are damped.

Resonance Pipe Experiment explained (similar concept) - Bennett Science:

📄 Harmonic Knives - The Naked Scientists: https://www.thenakedscientists.com/get-naked/experiments/harmonic-knives

• Try different types of cutlery (e.g., forks, spoons) to see how their vibrations differ.
• Use a tuning fork alongside the knife for comparison of harmonics.
• Record the sounds and use software to visualize the different frequencies produced.

• Use a blunt table knife, not a sharp one, to avoid injury.
• Strike the knife gently to prevent it slipping from your hand.
• Ensure you are not near delicate objects that could be damaged if the knife is dropped.

• Why does holding the knife at different points change the sound it makes? (Because holding at a node allows vibrations to continue, while holding at an antinode absorbs energy and damps vibrations.)
• What is a harmonic, and how does it relate to musical sounds? (Harmonics are higher-frequency vibrations of an object that combine to create complex sounds; they form the basis of musical tones.)
• Why does holding the knife at the very end stop nearly all vibrations? (Because all harmonics involve motion at the end, so gripping it there removes energy from every vibration mode.)
• How does this relate to the design of musical instruments like guitars or violins? (Musicians use nodes and antinodes when pressing strings to change pitch and control which harmonics are emphasized.)