# Decelerate is often used to indicate that an object is slowing down

1. Oct 9, 2007

### TonkaQD4

The term decelerate is often used to indicate that an object is slowing down. Does this term indicate the sign of the acceleration?

I believe the answer is no, but if someone could clarify why and how I would appreciate it.
Thanks

2. Oct 9, 2007

### Hootenanny

Staff Emeritus
I dissapprove of the term 'deceleration' since it is often very confusing, it is better to quote and appropriate vector acceleration. And to answer your question it depends how the deceleration is quoted.

Last edited: Oct 9, 2007
3. Oct 9, 2007

### TonkaQD4

So in a way the two terms are dependent of one another,correct? I am still a little confused on how to specifically answer this question that I posted? The term deceleration does not indicate the sign of acceleration, because a negative ( - ) acceleration can be speeding up in a positive direction which implies that the term deceleration can not be used as a negative acceleration. ?????

4. Oct 9, 2007

### CEL

If an object is decelerating, the dirtection of the acceleration is contrary to the direction of the velocity.

5. Oct 9, 2007

### robphy

It might be best to say that an object decelerates if the dot-product of its velocity-vector with its acceleration-vector is negative. That is,
$$\vec a \cdot \vec v < 0$$.
Since the left-hand side can be written as
$$\frac{d \vec v}{dt} \cdot \vec v =\frac{1}{2}\frac{d (\vec v \cdot \vec v)}{dt} = \frac{1}{2}\frac{d (| \vec v|^2)}{dt}$$,
then this says (as you say) that the speed is decreasing.

6. Oct 9, 2007

### saber1357

An object with a negative velocity that is decelerating is, in fact, speeding up. So the answer is no, or at least for my intro class it's no :)

7. Oct 9, 2007

### TonkaQD4

Thanks everyone