Aims

• Review core work on wave quantities and properties.
• Understand how a standing wave is the product of two identical waves travelling in opposite directions.
• State the differences between standing and progressive waves.
• Consider the wave in a guitar string as an example of a standing wave and understand how the physical properties of the string give rise to the notes played on the guitar.
• Consider closed and open pipes as examples of standing waves and understand why they have different harmonics.

TOK

• Why is the musical scale made up of such strange frequencies, wouldn't it be easier if it were decimalised?
• Is it important for musicians to understand about standing waves?
• If a musician understands the physics of the instrument does it make them a better musician or does it spoil the experience?

Notes

Explaining how two waves add together to give a standing wave is very difficult without being able to move the waves across the board. I used to use cardboard cut outs but now I use the smart board.

I show how the reflected wave looks by first using a long rubber tube and then with the waves in a string applet from phet. The applet is much better for showing the reflection but much more difficult to set up a standing wave. The reason is that you can't feel the wave coming back to your hand so its difficult to time the pulses correctly.

Listing the differences between standing waves and progressive waves is a common exam question so make sure you emphasise this.

Students often get confused when calculating λ for the different harmonics. I split the wave into ¼ wavelengths and count how many there in in the pipe/string.

Its always more fun to use real instruments than sonometers and glass tubes so ask your students to bring instruments into class and talk about the physics behind the different ways of playing. Get a student who is good at playing the guitar to demonstrate harmonics and tapping.

Test

Standing waves

Standing waves answers

  Standing waves mc test

Guitar problems

Text Book

Page 130 to 134

Homework

Problems 14 to 16

Vocabulary list

standing wave
progressive wave
harmonic
fundamental
woodwind
brass