When a body does not change its position with respect to time, then it is said to be at rest.
Motion is the change of position of an object with respect to time.

**Kinematics**

Mechanics is one of the oldest branches of
physics. It deals with the study of particles or bodies when they are at rest
or in motion. Modern research and development in the spacecraft design, its
automatic control, engine performance, electrical machines are highly dependent
upon the basic principles of mechanics. *Mechanics*
can be divided into *statics *and* dynamics*.

*Statics *is the study of objects at rest; this requires
the idea of* *forces in equilibrium.

*Dynamics
*is the study of moving objects. It comes from
the Greek* *word *dynamis* which means power. *Dynamics*
is further subdivided into *kinematics *and* kinetics*.

*Kinematics
*is the study of the relationship between
displacement,* *velocity, acceleration
and time of a given motion, without considering the forces that cause the
motion.

*Kinetics
*deals with the relationship between the motion
of bodies* *and forces acting on them.

We shall now discuss the various fundamental
definitions in kinematics.

*Particle*

*A
particle is ideally just a piece or a quantity of matter, having practically no
linear dimensions but only a position.*

*Rest and Motion*

*When a
body does not change its position with respect to time, then it is said to be
at rest.*

*Motion is the change of position of an object
with respect to time.*

To study the motion of the object, one has to
study the change in position (*x,y,z*
coordinates) of the object with respect to the surroundings. It may be noted
that the position of the object changes even due to the change in one, two or
all the three coordinates of the position of the objects with respect to time.
Thus motion can be classified into three types :

*(i) Motion in one dimension*

Motion of an object is said to be one
dimensional, if only one of the three coordinates specifying the position of
the object changes with respect to time. Example : An ant moving in a straight
line, running athlete, etc.

*(ii) Motion in two dimensions*

In this type, the motion is represented by any
two of the three coordinates. Example : a body moving in a plane.

*(iii) Motion in three dimensions*

Motion of a body is said to be three
dimensional, if all the three coordinates of the position of the body change
with respect to time.

Examples : motion of a flying bird, motion of a
kite in the sky, motion of a molecule, etc.

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