In the same way as we used velocity to characterize the way in which
position evolves with time, we introduce acceleration to
characterize how velocity evolves with time.
The average acceleration is defined as the average time-rate of increase of
velocity:
The more velocity changes in a given time unit the larger is the acceleration.
The dimension for acceleration is 
and the unit
is m/s
.
The average acceleration equals the slope of the line connecting
the point 
to 
. For our case of constant acceleration
we see that the average acceleration is independent of the time interval chosen.
We also introduce the instantaneous acceleration
For our particular case the a(t) curve is simple, just a
constant independent of time. In general, however,
there is no reason that acceleration should be constant.