Question

define acceleration and state its SI unit for motion along a straight line when do we considered the acceleration to be
1)positive
2)negative
give an example of a body in a uniform acceleration

Answer 1

"Rate of change of velocity is called acceleration"
SI UNIT: m/s2
>Acceleration is said to be positive if velocity of body is increasing and is said to be negative if velocity is decreasing.
e.g a body travelling in a circle has uniform acceleration.

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.