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# Progressive Waves

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## Progressive Waves

**Mechanical waves** are any waves that move through a medium. For example, water waves.

**Progressive waves** distribute energy from a point source to a surrounding area. They move energy in the form of vibrating particles or fields.

**There are two different types of progressive waves:**

**Transverse waves**- vibrations are perpendicular to the wave motion - so if the wave is travelling horizontally, the vibrations will be up and down. For example, light and water.**Longitudinal waves**- vibrations are parallel to the wave motion - so if the wave is travelling horizontally, the particles will be compressed closer together horizontally, or expanded horizontally as they go along (we call the expanded bit a rarefaction). The particle movement is a series of compressions and rarefactions. For example, sound and some earthquake waves.

**A Reminder of the Basics!**

Waves can be represented on distance or time graphs (* Note:* Look carefully at these graphs. They have different values on the x-axes):

This graph shows us how the displacement (s) of particles varies along a wave.

This graph shows us how the displacement of particles at a point varies with time. The time period of a wave can be found by measuring the time between two identical points along the wave.

**Displacement** is the distance a particle moves from its central equilibrium position.

**Amplitude** is the maximum displacement from the central equilibrium position.

**Phase angle** is the position along the wave, which is normally measured in degrees or radians. One complete wave is 360 degrees, so from a peak to a trough will be a change in phase of 180 degrees.

**We can calculate the speed of a wave using:**

**v = f λ**

**Where:**

**v** = speed (m/s)

**f** = frequency (Hz)

**λ** = wavelength (m)

You need to be able to derive this equation from speed = distance/time.

If the time for one complete wave is the **time period, T **and the distance is the **wavelength, λ,** then:

**Question:**