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Basic Concepts of Kinematics

In order to understand a physical situation happening around us, we involve idealized versions of the physical situations.

The study of a moving body becomes easy if we use the concept of the particle. A particle is a hypothetical mass point that has no size. A particle can undergo only translational motion. An object can be treated as a particle only when its size is small compared to either the distance traversed by the object or the distance separating it from other bodies with which it interacts.

Position and Coordinate System
While walking on the road, how are we able to distinguish one lamppost from the other? We notice that they are at different positions. Space is made of points and when a body occupies any one of these points in space, we call that point as the position of the body.

The position of an object is unique and in order to describe the position in a quantitative way we define ‘where the body is in space’ with respect to other bodies, i.e., we need a reference point to define the position of a body. In physics, we often draw a set of coordinate axes as shown in Figure 2.1 to represent the position. The reference point is the point of intersection of the axes and is called the origin. Such a set of co-ordinate axes is called a co-ordinate system.


The length of the path traversed by a body in a certain interval of time measured along the path is called the distance travelled by the body. It is a scalar quantity and is represented by the letter s. The unit of distance in SI and CGS system are metre (m) and centimetre (cm), respectively.

The shortest distance from the initial to the final position of a moving body is called the displacement. It is a vector quantity and is represented by the symbol Description: 47510.png. Its unit in SI and CGS system is metre (m) and centimetre (cm), respectively.

Difference Between Distance and Displacement

 Distance is a scalar quantity.
 Displacement is a vector quantity.
 Distance depends on the actual path  taken by the body during the whole  journey without taking into consideration  its direction.
 Displacement is the distance in a particular direction  which does not depend on the actual path of motion.  It depends only on the initial and final positions of the  body.
 Distance travelled is always greater than  or equal to the displacement.
 Displacement is always less than or equal to the  distance travelled. It can be equal to zero even if the  distance is not zero.






The average velocity of a body can be zero even if its average speed is not zero.
For example, suppose you take half an hour to come from home to school and another half an hour to go back home. If the total distance travelled is 5 km, your average speed is Description: 45737.png or 5 km h–1. But since you have finally reached your home, which was the starting point of your journey, your net displacement is zero. Hence, your average velocity is zero.

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