The velocity of a ball as it is thrown up is 20 meters/second. It attains zero velocity after 3.5 seconds. What is the average acceleration of the ball during its upward flight?

Change in speed = (speed at the end) minus (speed at the beginning)

= (0 - 20 m/s) = - 20 m/s

Acceleration = (change in speed) / (time for the change)

= (-20 m/s) / (3.5 sec) = - 5.71 m/s²

Notice that this story probably didn't happen on Earth, because

the acceleration due to gravity on Earth is 9.8 m/s².