oscillating in isolation

 we studied the motion of objects
oscillating in isolation. What happens in a system, which is
a collection of such objects? A material medium provides
such an example. Here, elastic forces bind the constituents
to each other and, therefore, the motion of one affects that of
the other. If you drop a little pebble in a pond of still water,
the water surface gets disturbed. The disturbance does not
remain confined to one place, but propagates outward along
a circle. If you continue dropping pebbles in the pond, you
see circles rapidly moving outward from the point where the
water surface is disturbed. It gives a feeling as if the water is
moving outward from the point of disturbance. If you put
some cork pieces on the disturbed surface, it is seen that
the cork pieces move up and down but do not move away
from the centre of disturbance. This shows that the water
mass does not flow outward with the circles, but rather a
moving disturbance is created. Similarly, when we speak,
the sound moves outward from us, without any flow of air
from one part of the medium to another. The disturbances
produced in air are much less obvious and only our ears or
a microphone can detect them. These patterns, which move
without the actual physical transfer or flow of matter as a
whole, are called waves. In this Chapter, we will study such
waves.