as my high school physics teacher stressed, deceleration does not exist. it was a forbidden word in my introductory physics class. I do indeed partly agree with my teacher. Since if deceleration is a vector quantity, it would be very confusing since deceleration sort of implies the "negative" of acceleration. In that case, deceleration can simply be replaced by an acceleration vector in the opposite direction. perhaps it would make more sense to define deceleration as the decrease of the speed. just like how molecule decelerate when their temperature decrease. However, it is still confusing when one switch frame of reference, where the decrease in speed in one frame can be an increase in speed in another. well... the discussion of deceleration is indeed quite silly... but what are your thoughts?