The  most basic and obvious result of physical interactions in motion: a brick falls, an automobile moves up a hill, an ear drum vibrates, a compass needle swings into line with the magnetic field, a radioactive nucleus emits a beta-particle and so on. Most of our understanding of nature is deduced from our observations of motions and our efforts to relate them to their causes. Therefore, let us beign our study of physics by developing the ideas needed for discussion of motion starting with the cause of an object moving in a straight line.

Mechanics deals with the behaviour of various types of bodies such as particles, rigid bodies and fluids, when subjected to the action of force. Mechanics of particles and rigid bodies is subdivided into dynamics, statics and kinematics. Dynamics is the study of motion in relation to the forces that cause motion. Statics is the study of bodies at rest and the conditions under which they remain at rest. Kinematics is the study of the motion of bodies without regard to the cause of that motion.

DESCRIPTION OF MOTION

The following are some of the terms generally used in the description of motion.

BODY: A body is a portion of matter which is infinitesimally small. It is so small that the distance between its difrrent parts may be neglected. It is often regarded as a mathematical point endowed with mass.

Although a body is composed of a large number of particles, we still regard a body to be a particle. The dimensions of the body are negleted and assume it to be a mathematical point at which the entire mass of the body is concentrated.

REST: A particle is said to be at rest if it does not change its position with respect to its surroundings.

MOTIONS: A particle is said to be in motion if it changes its position with respect to its surroundings.

PATH: Path of a particle is a line drawn through succesive positions of the paticle during motion.

SPEED: Objects in motion travel certain distances in given time. An automobile, for example, travels so many kilometers in one hour. Speed is a measure of how fast something is moving.

Speed of a particle is defined as the rate if change of position of a particle without refrence to direction.

therefore, Speed = Distance travelled / Time taken

Speed is a scalar quantity and is measured in meter per second(ms-1). The dimensions of speed are (m0 l1 t-1).

AVERAGE SPEED: The average speed of a particle in a given interval of time is the speed of moving particle that moves with uniform speed and desribes the same distance as the particle in the same time.

The average speed is defined as follows:

Average speed = Total distance travlled / Timetaken

Since average speed is the whole distance covered by total time taken, it does not indicate diffrent speeds and variations that may have taken place. In practice we experince a variety of speeds on most journeys. So average speed is often diffrent from the instantaneous speed.

DISTANCE AND DISPLACEMENT: The total length of the path traveresed by a body during certain time interval called the distance. It is a scalar quantity and is measured in metre.

Displacment is defined as the distance moved in a given direction displacment is a measure of the diffrence in positions between two points. The magnitude of this vector quantity is the straigth line path between these two points. The direction of displacment is the direction from one of these points to the other.

VELOCITY: The rate of change of position in a given direction is called velocity,thus

Velocity = Distance travelled in a given direction / Time taken

Velocity is a vector quantity and it is measured in meter per second(ms-1). The dimensions of velocity are (m0 l-1 t-1).

Velocity of a particle may be uniform or variable is said to be uniform if it travels along a straight line, covering equal distances in equal intervals of time, however small the intervals may be. Velocity of a particle is said to be variable if it moves along a straight line covering unequal distances in equal intervals of time, however small the intervals may be.