# Wave motion

A wave motion is the transmission of energy from one
place to another through a material or a vacuum. Wave motion may occur in many forms
such as water waves, sound waves, radio waves and light waves, but the waves are
basically of only two types:

transverse waves - the oscillation is at right angles to the
direction of propagation of the wave (Figure 1(a)). Examples of this type are water waves and
most electromagnetic waves.

(b) longitudinal waves - the oscillation is along
the direction of propagation of the wave (Figure 1(b)). An example of this type is sound
waves.

## Basic
definitions:

**Wavelength:** |
the distance between any two successive
corresponding points on the wave, for example that between two maxima or two minima (l) |

**Displacement:** |
the distance from the mean, central, undisturbed (y) |

**Amplitude:** |
the maximum displacement (a) |

**Frequency:** |
the number of vibrations per second made by the wave (f) |

**Period:** |
the
time taken for one complete oscillation (T= 1/f) |

**Phase:** |
a term related to the displacement
at zero time (e) |

(see 16-19/Wave properties/Wave properties/Text/Phase change)
We will
consider here the motion of a sine wave (Figure 2), since this type is the most fundamental.
However it can be shown that any other wave may be built up from a series of sine waves of
differing frequency.

We can express a wave travelling in the positive x
direction by the equation:

Positive x direction : y = a sin(wt – kx)
and
for one travelling in the opposite direction:

Negative x direction : y = a sin(wt – kx)
where k is a constant and

w =
2

pf.

The sign gives the direction of the motion. We can
separate each equation into two terms:

(a) a term showing the variation of displacement
with time at a particular place - for example, when x = 0 y = a sin (

wt), that is, the variation of displacement with time at the particular
place x = 0.

(b) a term showing the variation of displacement with distance at a particular
time - for example, when t = 0 y = a sin (kx), that is, the variation of displacement with
distance at a particular time t = 0.

An alternative form of the equation can be proved
as follows.

Since the period T = 1/f where f is the frequency and

w = 2

pf we have

w = 2

p/T.

Also when t = 0 y = 0 at x =
0,

l/2,

l...and so on, and so k =
2

pl. The equation may therefore be
written:

y = a sin 2p(t/T + x/l)
**Example problem**

A certain travelling wave has frequency (f) of 200 Hz, a wavelength (l) of 2m and an amplitude (a) of 0.02 m.

Calculate the displacement (y) at a point 0.3m from the origin at a time 0.01s after zero displacement at that point.

The period of the wave = 1/f = 1/200 = 0.005 s^{-1}

y = a sin 2p(t/T + x/l) = 0.02 sin[2p(0.01/0.005 + 0.3/2)]

= 0.02 sin[2p(2 + 0.15)] = 0.02 sin[13.5] = 0.02x0.81 = 0.016 m