Physics class Eight : Wave

Progressive Waves

 

Types of 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.

Calculating the Speed of a Wave

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:

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