"Acceleration" doesn't mean "speeding up".
It means ANY change in speed OR direction of motion.
A). The race car is moving at 150 mph. Speed isn't changing, and we don't
know anything about its direction. No evidence of acceleration there.
B). A sailboat moves west, then north.
It changed direction. During the time that it was sailing
on a curve or around a corner, it was accelerating.
C). A train half way between New York and Houston.
We don't know anything about its speed OR direction right now.
No evidence of acceleration there.
D). A book sitting motionless.
Its speed is zero. We know for a fact that its speed and direction
are not changing. So there is definitely no acceleration here.
