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# Kinematics - 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.

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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