Acceleration is defined as the change in velocity with respect to time. Acceleration is vector quantity. The SI unit of acceleration is m/s^{2}.

Acceleration = Change in velocity/time

If the initial velocity is ‘u’, and after time ‘t’ the velocity is final velocity. So change in velocity is, final velocity – Initial velocity.

Acceleration (a) = Final velocity – Initial velocity/time

∴ a = v-u/t

__E.g: __

(1) Slide in the park. While sliding down velocity is less at top increases at top increases of middle and zero at end.

(2) The fruits is falling down.

**Numericals on Acceleration Formula**

**A bus slows down from a speed of 68 km/h to reset in 20 seconds. What is acceleration of bus in m/s ^{2}. **

Ans: Initial velocity ‘u’ 68 km/h

final velocity ‘v’ = 0 km/h

convert 68 km/h into m/s

1 km = 1000 m

So, 1000/20 = 50 m/s

Initial velocity ‘u’ = 68 km/h

= 68 × 50 m/s

= 3400 m/s

Final velocity ‘v’ = 0 km/h

= 0 × 50 = 0 m/s

Now acceleration,

Acceleration = Final velocity – initial velocity /time

= v – u/t

= 0 – 3400/20

= – 3400/20 = -170 m/s^{2}

Acceleration = -170 m/s^{2}

Here velocity is decreases with time and direction is opposite to direction of velocity.