# Does deceleration make sense?

## Main Question or Discussion Point

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?

## Answers and Replies

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jtbell
Mentor
Imagine yourself sitting in a car with your eyes closed. Can you tell the difference between moving forward with increasing speed ("accelerating"), and moving backward with decreasing speed ("decelerating")?

rcgldr
Homework Helper
If you have a frame of reference for speed / kinetic energy, then I would define acceleration as the one that increases velocity / kinetic energy, and deceleration as the one that decreases speed / kinetic energy.

I guess technically deceleration does not exist as an entity in itself, but it is just a descriptive word. As was stated, deceleration is just acceleration in the opposite direction. What happens when you finally reach a velocity of 0 and start accelerating in the opposite direction of where you were originally going? Your acceleration/deceleration has not changed; surely you are not decelerating anymore, but you have been accelerating in the same direction the entire time.

However, I believe most people understand the concept of deceleration so it is silly to forbid its use . It's like forbidding the use of the word cold: There is no such thing as scientific cold; it is just a lack of heat, but everyone knows what cold describes.
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radou
Homework Helper
Deceleration is a relative term, that's all.