Acceleration is the change in velocity with time.
Formula: a = (v - u)⁄t
where v = final velocity,
1. Positive acceleration: change in velocity is in the same direction as the velocity.
=> Speed is increasing.
2. Negative acceleration: change in velocity is in the opposite direction to the velocity
=> Speed is decreasing.
=> Also known as deceleration or retardation
(1) If the direction of the object changes, it is undergoing acceleration as change in direction = change in velocity = acceleration.
(2) Decreasing acceleration ≠ deceleration. It means increasing speed at decreasing rate.