# What does it mean to say that an object decelerates?

1. Sep 16, 2015

### wein7145

1. The problem statement, all variables and given/known data
What does it mean to say that an object undergoes a deceleration? Can this term be used without ambiguity? Edit (That was straight from the book poorly written: Should say object decelerates not undergoes a deceleration)

2. Relevant equations
v(t)=v0+a⋅t

3. The attempt at a solution
Deceleration means less acceleration or going slower. But this can be ambiguous in terms of positive and negative: it can be a little less positive but still positive or a little less negative but still negative this same problem happens in velocity.

I just want a second opinion on this the textbook does say something about asking questions and making conclusions that better a larger system of Physics. This larger teamwork based system of Physics is how it all works at our jobs apparently which is why we are learning this way; to be better prepared for our future jobs. It makes sense I can't just make up laws or rules without knowing what other people agree on/what the real accepted laws or rules are.

2. Sep 16, 2015

### axmls

Do you have any reason to believe that an object which goes from an acceleration of 10 m/s^2 to 9 m/s^2 has decelerated? After all, it's still speeding up.

3. Sep 16, 2015

### haruspex

Those are two different things. One is right, one is wrong.

4. Sep 16, 2015

### wein7145

I'm pretty sure that it decelerated by 1m/s yes it still is positive but decelerated. 10m/s to 11m/s is acceleration by 1m/s.

Oh...which one is right the book says something about acceleration is positive if velocity over time is more positive or less negative than initial velocity and acceleration is negative (deceleration?) if velocity over time is more negative or less postive than initial velocity

5. Sep 16, 2015

### haruspex

Read axmls' post more carefully. 10m/s is a speed; axmls wrote 10m/s2, an acceleration.
No, I don't think deceleration is the same as negative acceleration. It's tricky because there's a difference between everyday usage and technical definition.
In physics, acceleration is a vector. Expressed in a co-ordinate system, it may be negative with respect to an axis. If an object is moving in the positive x direction with a negative acceleration then it is slowing down. If it is moving in the negative x direction with a negative acceleration it is getting faster.
The term 'deceleration' has no use here. All changes in velocity (including changes of direction) are accelerations. It is only used, as far as I am aware, in a scalar sense. That is, as a reduction in speed.

6. Sep 16, 2015

### FG_313

You are using the wrong dimensions for acceleration, insted of m/s, acceleration is measured by m/s^2. An acceleration is a rate of change of velocity, in relation to time. In some time to there is a velocity vo, and for a time later on t1 there is a velocity v1. The average acceleration of an object is defined as (v1-vo ) / (t1-vo). In that way, if v1>vo, there is a positive acceleration. If v1<vo, there is a negative acceleration. Depending if you're dealing with scalars or vectors, this can mean different things sometimes. But dealing with scalars, if there is a positive acceleration, than the object gains speed. If there is a negative acceleration and the object looses speed, than it's called a deceleration.

7. Sep 16, 2015

### wein7145

OH Whoops yeah that's right m/s is speed and m/s^2 is acceleration. But back to the main question
Maybe that is what the book is trying to teach I understand that. Which means there is ambiguity yes that answers part of the question.
That answers the other part of the question. Thanks for the help.