# B Direction of acceleration, vector quantity

1. Oct 9, 2016

### Kajan thana

Hi guys,

If a particle is accelerating to the left, does this means the particle is decelerating to the right? And also does the sign change from negative to positive?

Many Thanks.

2. Oct 9, 2016

### Staff: Mentor

Sure. If you swap the sign of the coordinates, then velocity and acceleration change their sign as well.

3. Oct 9, 2016

### David Lewis

My understanding is that deceleration is not a defined term in physics.
Technically, when an object slows down, it accelerates.

4. Oct 10, 2016

### Staff: Mentor

Acceleration that lowers the speed of the object is often called deceleration.

5. Oct 10, 2016

### A.T.

If it's moving to the left, and accelerating to the right, then it is decelerating. "Decelerating to the right" doesn't make sense because deceleration is the rate of reduction of speed which is a positive scalar.

6. Oct 10, 2016

### sophiecentaur

Yes, in everyday conversation but not by Physicists or Mathematicians , in the context of their work. Magnitude and Sign are far more useful than two separate terms. How could you formulate or apply the basic formula for motion under constant acceleration if you had to change the name of what you are dealing with, once the force direction changes?
The Acceleration / Deceleration thing is just one of many pairs of descriptors in common use. Hot / Cold , In / out, Up / down, screw / unscrew etc. To do any useful calculation, the first thing you have to do is to abandon such terms of description.

7. Oct 10, 2016

### A.T.

The "acceleration" above (speed increase) is different from acceleration in physics which encompasses all velocity changes. That's the confusing aspect of bringing up "deceleration".