What is Kinematics? // Motion and Rest position of the Objects // What is Distance and Displacement?


In physics motion and rest are relative terms. These depend upon the situation of the observer and the object being observed. The two persons travelling on a moving bus are at rest with respect to each other but are in motion with respect to observers standing on the highway. So rest and motion can be determined in terms of the observer. If a body changes its position with respect to the observer, then the body is in motion, otherwise it is at rest.

A body may remain at rest when no force acts upon it. But a body can also be at rest under the action of zero net force and therefore that branch of mechanics which deals with body at rest is called static.

The study of motion can be treated in two ways. 

(i) Kinematics: It is the branch of mechanics which deals with the study of the motion of bodies without taking into the account of the cause of the motion in the bodies. 

(ii) Dynamics: This branch deals with the study of motion of the objects taking into consideration the causes of motion. 

2.Terms Regarding Motion 

The length of actual path followed by an object between its initial and final position is called the distance. It's magnitude does not remain constant and is always a positive quantity.
Figure: Distance and displacement between P and Q.

The shortest length of the path between two points in a direction is called displacement. So its magnitudes is fixed and can be positive, negative or zero. So displacement is the distance moved in the given direction.
Path 1, 2 and 3 in above figure
 all represent the distance where as path 2 only represents displacement. So the distance is greater or equal than the magnitude of displacement, (distance > displacement).
Displacement is a vector quantity and its unit is meter in SI-units.