pic from citejournal( Fig.6)
A bouncing ping pong ball spend most of its time in the air , whether on the way up or down its acceleration due to gravity act downwards and has the unchanging value of 9.81 ms-2.Taking downwards as negative the acceleration time graph (graph 3) is a horizontal straight line (at -ve value) except at a very short time interval where it suffers a smack from the ground in a direction opposite to its weight and this explains the peak found in the graph 3. Obviously this force is large compared to its weight and thus the acceleration is positive and has a large magnitude. As it bounces a number of times its height decreases and the upward force decreases and the +ve acceleration decreases in magnitude too. As the height decreases the time spend in the air is shorter and the horizontal line showing -ve 9.81 ms-2 should get shorter (see graph 3 above).
