Acceleration - Uniformly acceleration Motion
The velocity of the ball may change by changing the direction, or by changing both magnitude and direction.
This variation of velocity divided by the time elapsed is the average acceleration.
The average acceleration of a ball is defined by:
Average acceleration = change velocity/ time elapsed
Example: a = 
v / t = 50 (m/s) / 5.0 s =
10.0 m/s2
This value of acceleration means that in each second the velocity changed 10.0 m/s.
An uniformly acceleration motion is in a straight line in which the direction is always the same and the velocity changes at a constant rate, i.e., the acceleration is constant.
Freely falling ball is an example of uniformly accelerated motion, when the air resistance is negligible. The ball is falling under action of its weight alone.
The acceleration of freely falling ball (fig. 1) is named acceleration of gravity and is represented by the symbol g. At sea level and 45o latitude, g has a value of 9.8 m/s2.
Example:
Figure 1 - Motion freely falling of ball ping-pong
See this image in avi: ping-pong.avi
Have you ever use your finger to press down one side of the ping-pong ball? If you press hard enough, you will find ping-pong
ball running away from you. However, a few second later, the ping-pong starts to rolling
back toward you. Why?
See the answer in this site: http://webphysics.ph.msstate.edu/javamirror/ntnujava/rotateDisk/pingpong.html