# Mechanical and electromagnetic waves

Question:
What is the difference between mechanical waves and electromagnetic waves?
Are the following either a continuous wave, a pulse or neither

- a singer holding a high note for a long time
- an explosion
- dominoes falling

Mechanical waves
These would be set up within a solid, a liquid or a gas due to the vibration of the molecules there. They are transmitted by both intermolecular forces and by collisions between the molecules. In solids they could be either longitudinal or transverse vibrations while in a liquid or a gas only longitudinal vibrations are really possible. Examples of these waves would be the P and S waves in the Earth's crust due to an earthquake and sound waves in air.

Electromagnetic waves
These do not require a medium for their transmission. They are made from an electric and a magnetic field oscillating at right angles to each other. Examples of these would be light, X rays, ultra violet, microwaves etc.

Now the pulsing or non-pulsing of your examples.

(a) a singer – this is a continuous wave train. You can see this by recording the sound of a human voice and loading into a computer sound analysis program (I have used a good one in England produced by Cambridge Scientific Media). The vibrato in the voice does not break the wave train, only change the loudness and maybe the pitch slightly.
(b) an explosion – the explosion is also a continuous wave train but one of definitely varying amplitude. After the initial loud sound the amplitude dies away but once again the wave train is not broken (see the sound analysis program again).
(c) falling dominoes – this is rather like a series of small explosions as each domino falls over. What I said about the explosion applies to each domino but the effect of a whole series of them falling over will be to given a broken wave train and so a series of pulses. These pulses may well overlap. Each time a domino hits the ground a pulse will be generated and this will die away as time passes.