Define acceleration and state its SI unit. For motion along a straight line, when do we consider the acceleration to be (i) positive (ii) negative? Give example of a body in uniform acceleration.
Acceleration: Acceleration of a body is defined as the rate of change of its velocity with time
Acceleration = 
SI unit: ms-2
For a body moving in a straight line we consider it to be moving with
a) positive acceleration if the speed increases with time
b) negative acceleration if speed decreases with time
An example of uniform acceleration is: a ball rolling down a frictionless inclined plane.
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