Mechanical wave motion
two types of mechanical wave motion are (i) transverse wave motion and (ii)
longitudinal wave motion
(i) Transverse wave
Transverse wave motion is that wave motion in which particles of
the medium execute SHM about their mean positions in a direction perpendicular
to the direction of propagation of the wave. Such waves are called transverse
of transverse waves are waves produced
by plucked strings of veena, sitar or violin and electromagnetic waves.
Transverse waves travel in the form of crests and troughs. The maximum displacement of
the particle in the positive direction i.e. above its mean position is called
crest and maximum displacement of the particle in the negative direction i.e
below its mean position is called trough. Thus if ABCDEFG is a transverse wave, the points B and F are
crests while D is trough (Fig. ).
waves, the medium must
possess force of cohesion and volume elasticity. Since gases and liquids do not
have rigidity (cohesion), transverse waves
cannot be produced in
gases and liquids. Transverse waves can be produced in solids and surfaces of
(ii) Longitudinal wave
?Longitudinal wave motion is that wave motion in which each particle
of the medium executes simple harmonic
motion about its mean position along the direction of propagation of the wave.?
Sound waves in fluids
(liquids and gases) are examples of longitudinal wave. When a longitudinal wave
travels through a medium, it produces compressions and rarefactions.
the case of a spiral spring, whose one end is tied to a hook of a wall and the
other end is moved forward and backward, the coils of the spring vibrate about
their original position along the length of the spring and longitudinal waves
propagate through the spring (Fig.).
regions where the coils are closer are said to be in the state of compression,
while the regions where the coils are farther are said to be in the state of
When we strike a
tuning fork on a rubber pad, the prongs of the tuning fork begin to vibrate to
and fro about their mean positions. When the prong A moves outwards to A1,
it compresses the layer of air in its neighbourhood. As the compressed layer
moves forward it compresses the next layer and a wave of compression passes
through air. But when the prong moves inwards to A2, the particles
of the medium which moved to the right, now move backward to the left due to
elasticity of air. This gives rise to rarefaction.
Thus a longitudinal
wave is characterised by the formation of compressions and rarefactions
following each other.
Longitudinal waves can
be produced in all types of material medium, solids, liquids and gases. The
density and pressure of the medium in the region of compression are more than
that in the region of rarefaction.