Introduction
The rock itself is on a planet spinning about an axis
through its centre and whirling about another axis through the centre of a
galaxy. The galaxy is itself streaming apart from countless other galaxies in
the Universe. The motion of every piece of matter in the Cosmos is controlled
by influences that we call forces. The origins of some forces are
readily visualised. If two large isolated lumps of matter are connected by a
thin stretched spring in the far reaches of outer space and released from rest
then they are attracted towards each other as the spring relaxes. The motion of
each lump is characterised by its instantaneous velocity and acceleration and
may be measured by timing its displacement. The ratio of the accelerations of
the two lumps of matter depends on how much stuff is contained in each lump.
Indeed if different masses are attached to the same spring this ratio is found
to depend only on a property of each lump that we call its inertial
mass and the lump with the largest mass is observed to exhibit the
smallest acceleration.
The spring is said to exert a contact force on
each lump and if unrestrained responds to the force by moving. Most springs
have a natural length for which they exhibit zero contact force. As long as the
spring is longer than its natural length it is said to be in tension
and if connected to lumps of matter attempts to pull them together. As the
lumps approach each other the tension in the spring decreases and will
eventually become zero. By this stage however the lumps are moving towards each
other and continue to move thereby decreasing the length of the spring below
its natural length. The spring is then said to be compressed. It
transmits a compressive contact force to each lump that retards their
motion eventually bring them instantaneously to rest and then drives them
apart. Now the length of the spring begins to increase and eventually become
larger than its naturally length. The repulsive phase of the motion becomes
arrested by the "tensile" contact force that is now in operation and the
attractive phase starts a repeat of the whole cycle. This is perhaps the
simplest example of an oscillating system consisting of two lumps of matter
connected by an elastic spring. Its evolution in time is an example of a
vibrating system.