# Physics acceleration and negative decelestion

Please could someone explain to me the difference between the two? Thanks
Do you mean deceleration?
Original post by Sinnoh
Do you mean deceleration?

Yes
Original post by Ashirs
Yes

The word "acceleration" is applicable to any situation where velocity is changing with time, whether velocity is increasing or decreasing.

"Deceleration" is typically used when there is a reduction in velocity. In that case, an acceleration of -2 m/s^2 would be the same as a deceleration of 2 m/s^2.

Probably better to just keep it all in terms of acceleration, that way you're less likely to make an error with signs.
Original post by Sinnoh
The word "acceleration" is applicable to any situation where velocity is changing with time, whether velocity is increasing or decreasing.

"Deceleration" is typically used when there is a reduction in velocity. In that case, an acceleration of -2 m/s^2 would be the same as a deceleration of 2 m/s^2.

Probably better to just keep it all in terms of acceleration, that way you're less likely to make an error with signs.

So is negative acceleration deceleration or acceleration backwards?
they are the same thing. by acceleration backwards, I presume you mean acceleration in the opposite direction.

Original post by Ashirs
So is negative acceleration deceleration or acceleration backwards?
Original post by iTsJames123
they are the same thing. by acceleration backwards, I presume you mean acceleration in the opposite direction.

Yes, but is deceleration not acceleration but slowing down?
Original post by Ashirs
Yes, but is deceleration not acceleration but slowing down?

yeah - but don't forget the A in SUVAT stands for acceleration so in order to use SUVAT and get the right answer if you're given deceleration you'd convert it to acceleration... by flipping positive deceleration to negative acceleration.
Original post by Ashirs
Yes, but is deceleration not acceleration but slowing down?

Deceleration is a type of acceleration.
However, I think it would be best to avoid using the term "deceleration" altogether. It can lead to confusion.
Original post by Ashirs
So is negative acceleration deceleration or acceleration backwards?

Negative acceleration is just another word for deceleration and it is in opposite direction to acceleration. Because of these different directions, the acceleration has another effect on the movements of objects compared to deceleration. Acceleration increases, while deceleration decreases the velocity.
(edited 5 months ago)
Original post by Ashirs
So is negative acceleration deceleration or acceleration backwards?

Before answering this question, let us see what is negative acceleration.
In kinematics or solving kinematics problems, you need to choose the direction of motion or travel to be positive.
In the diagram below, moving to the right is taken to be positive.

We can see that when the acceleration is negative, the car slows down or the velocity decreases. This is deceleration.
So it seems that negative acceleration means deceleration. No No No ...
See the diagram below for a more complete description.

We see “new” situation 3 where there is a negative acceleration but it is placed under the label of “Speeding up” not “Slowing down”.
For situation 3, the car is moving in the negative direction, so the velocity is also negative. The velocity becomes more negative with time, so the acceleration is negative. There is a decrease in velocity NOT an increase in velocity. But the car is speeding up not slowing down. Why?
I would answer the quote question first.
So is negative acceleration deceleration or acceleration backwards?

This is an invalid question because there is no context of the velocity.

Why? I shall answer this now.
In order to interpret the algebraic sign of acceleration correctly, we need to know the sign of velocity.
See the second diagram above to compare situation 1 and 2:
When both velocity and acceleration have the same sign or the velocity vector and acceleration vector are pointing in the same direction, the object is speeding up.
When both velocity and acceleration have a different sign or the velocity vector and acceleration vector are pointing in opposite directions, the object is slowing down.

Using the above interpretation of the sign of acceleration in relation to that of velocity allows us to see that in situation 3, the car is speeding not slowing down while the car is slowing down in situation 4.

In conclusion, slowing down can have either positive or negative acceleration and speeding up can have either positive or negative acceleration, too. Asking what is negative acceleration is an invalid or senseless question.

Avoid saying the following:
Deceleration is the reduction of velocity. (See situation 3)
Deceleration is negative acceleration. (See situation 3)
Acceleration increases velocity while deceleration decreases velocity. (See situation 3 and 4)

Best to avoid using the ambiguous term deceleration to self-confuse yourself.