Here we have a vehicle, that accelerates, decelerates suddenly, gains a little more speed and then decelerates until it stops.
[The x axis is for the time in seconds!, t(s) the photo didn't encompass it]
There are two moments that drew my attention: '$a$ to $b$' or '$c$ to $d$'
In 'a to b', we have a very sudden acceleration $a$ (which is working against the speed) that costs our vehicle 10m/s in only 0.5 second. Which gives a acceleration ($a$) of 20m/s^2. Here we have the greatest modulus for $a$ (acceleration) in the entire trajectory.
On the other hand, during 'c to d', we have a much more modest $a$ (acceleration), also working against the speed. But it'll work longer resulting in a greater ∆v (V-V0), the vehicle loses a total of 30m/s and stops, but it takes 5,5 seconds to do so.
I'd like to know, by definition, which would be the greatest deceleration:
The first momment ('a to b') in which we have a more sudden change in speed ($a$ has the greatest modulus working against speed), but it doesn't result in a loss of speed as great as the second: 'only' 10m/s
The second case ('c to d'), which encompasses the greatest loss in speed: 30m/s (v-v0), eventually stopping the car but taking more time to do so.
In other words, which defines the greatest deceleration in an interval of time? The relative deceleration, caused by how fast it makes the vehicle lose speed. Or an absolute deceleration, caused by how much speed one vehicle loses regardless of time.
In other words, is deceleration to be interpreted as the dictionary suggests:
Oxford Dictionary: The reduction in speed or reduction in rate. Speed: Speed brakes enable the aircraft to carry out rapid deceleration Rate: a deceleration in econimic growth
Or is deceleration interpreted as the rate of reduction in speed.