Physics Definitions (Ch-2)

Motion

When a body continuously changes it's position with respect to it's surroundings it is said to be in motion .

Rest:

 If a body does not change it's position with respect to it's surroundings it is said to be at rest . State of rest or of motion is relative . It means that a body at rest with respect to a certain frame of reference may be in motion with respect to some other frame of reference.

Displacement: 

Displacement of a body may be defined as : 

       "The change in position of a body in a particular direction".

It may also be defined as : 

       "The shortest straight line distance between initial and final positions of a body ". 

Displacement is a vector quantity. 

It's magnitude is equal to the distance between the two positions of the body , it is directed towards the final position . 

Speed:

 "Distance covered per unit time is called speed of the body". Speed is a scalar quantity. It's unit in M.K.S system is meter per second (m/s) .

Average speed :

 "Total distance covered divided by total time taken is called average speed ".

Instantaneous speed: 

                                        "It is the a body at a particular instant".
 Instantaneous speed of a body can be calculated by finding the distance covered by it in a very short interval of time and then dividing it by the time interval . Mathematically instantaneous speed is given by: 

It means that the interval of time is very taken is very short and is close to zero