1. 2-4 Acceleration (VECTOR)
Acceleration is…
the rate of change of velocity.

2. Acceleration is a vector, although in one-dimensional motion we only need the sign.
The previous image shows positive acceleration; here is negative acceleration:

3. Difference between negative acceleration and deceleration (I will not be picky about this):
Negative acceleration is acceleration in the negative direction as defined by the coordinate system.
Deceleration occurs when the acceleration is opposite in direction to the velocity.

4. 2-4 Instantaneous Acceleration
(2-5)
Example (board or OneNote):
A particle is moving with a velocity of 60.0 m/s in the
positive x direction at t=0. Between t=0 and t=15.0 s, the velocity decreases uniformly to zero. What was the acceleration during this 15.0-s interval? What is the significance of the sign of your answer?