XinyanWang said:
You mean Instagram conversation?Right
So your text is in the blue/mauve bubbles and your interlocutor's (the other person's) is in the grey bubbles, yes?
Then broadly I agree with you. The other person is confusing magnitude and vector concepts.
It doesn't help that many books get it a bit wrong, describing speed as a scalar. No, scalars are just like 1D vectors, so have a sign. Speed is a magnitude.
In referring to speed in everyday English, an acceleration is an increase in magnitude and deceleration is a reduction in magnitude. In this usage, there is the further ambiguity that acceleration may mean the amount by which the speed increased: "the car accelerated from rest to 30kph", or the
rate at which it increased.
But in physics we only use "acceleration" to mean a vector and there is no such concept as deceleration. The extract from "A level physics..." underlined in red, "
A negative acceleration is called a deceleration", is nonsense.
And in physics we only use "acceleration" to refer to the
rate of change. At least, I would only use it that way, so I have to disagree with this in post @2:
DrClaude said:
acceleration is the change of velocity
I also object to this statement in the next textbook extract:
"
Acceleration is defined as the rate of change of velocity. Therefore it must include the direction in which the speed is changing."
Obviously it should say "
the direction in which the velocity is changing."
Even so, there is still an ambiguity. There is average acceleration over an interval of time, ##\vec a_{avg}=\frac{\Delta \vec v}{\Delta t}##, and instantaneous acceleration, the limit of the average acceleration as ##\Delta t## tends to zero. The textbook should make that clearer.