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
