Question

why acceleration is taken as velocity upon time but not velocity upon distance​

Answer 1

because acceleration means.. the rate of change of velocity (change in speed in particular direction )of the object with respect to time.... thus acceleration formula is

$= \frac{v - u}{t}$

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Answer 2

Acceleration meant that rate of change of velocity..

And as well as it is a vector quantity which had. Magnitude and direction both .

Hence, the acceleration is taken as velocity upon time.