Answer
Toss a ball up in the air
Work Step by Step
Toss a ball straight up in the air. With an initial velocity (actually, it doesn't have to be straight up, it can be tossed at an angle, but that complicates this story a little)
Take up to be (+y).
(a) On its way up, the ball has the initial velocity gradually decreasing (but still positive) until it reaches its highest point. It is accelerating (decrease is change), and the acceleration vector points down, so the acceleration is negative.
(b) Once it reaches the highest point, where its instantaneous velocity is zero, from there onward, it begins increasing its velocity, towards the ground, therefore negative. Free fall acceleration still points down as before, so it is negative also.
