Question

define acceleration and state it's SI unit. for motion along a straight line, when do we consider the acceleration to be

Answer 1

the rate of change of velocity per unit of time is known as acceleration .

S.I unit of acceleration is m/s²
 

when the body have some non uniform velocity than it consider to doing acceleration in straight line motion 

Answer 2

Acceleration is defined as the rate of change in velocity.

Its S.I. unit is m/s^2

For motion along a straight line, if the velocity of the body is increasing with time, then it is said to be in positive acceleration. While, if the velocity of the body is decreasing with time, then it is said to be in negative acceleration or retardation.

The motion of a freely falling body and a vertically thrown up body are the  example of uniform accelaration.