# Introduction

Two main branches of mechanics are Statics and Dynamics. Statics is the study of the objects at rest and Dynamics is the study of objects in motion. Further, an object can be at rest or it can have uniform motion, even when a number of forces are acting on it. Such forces are said to be in equilibrium. Thus, statics is the study of the motion of an object under the effect of forces in equilibrium.

The motion of objects is studied under two separate headings:

Kinematics
The study of the motion of the objects without taking into account the cause of their motion is called kinematics.

Dynamics
The study of the motion of the objects by taking into account the cause (or causes) of their change of state (rest or of uniform motion) is called dynamics.

The various aspects of motion of an object can be understood in terms of a few physical quantities such as displacement, velocity, acceleration and time.

Galileo was the first to discover that the motion of an object can be described in terms of two fundamental physical quantities namely (i) length and (ii) time and two derived physical quantities namely (i) velocity and (ii) acceleration.

Objects in Motion
Rest and motion are relative terms. An object is said to be in motion, if it changes its position with reference to its surroundings with the passage of time. On the other hand, if an object does not change its position with reference to its surroundings with the passage of time, it is said to be at rest.

The Concept of a Point Object
While studying the motion of an object, sometimes, its dimensions are of no importance. For example, if one travels from one place to another distant place by a bus, the length of the bus may be ignored as compared to the distance traveled. In other words, although the bus has a finite size, yet for study of the motion of the bus along the road, its motion may be considered as the motion of a point particle.
In mechanics, a particle is a geometrical mass point or a material body of negligible dimensions. It is only a mathematical idealization. In practice, the nearest approach to a particle is a body, whose size is much smaller than the distance or the length measurements involved.