When we launch a ball at modest speed
vertically into the air,
it will eventually return back to us due to
gravity.
As an example, let us consider a solid metal ball
which is thrown upward into the air with a speed of 20 m/s,
equivalent to 72 km/h.
Below you find some measurements about its height
above the position where it is launched
at certain instants of time after it has been launched.
instant of time (s)
height of ball (m)
0.0
0.0
1.0
15.10
2.0
20.40
3.0
15.89
4.0
1.58
We observe that the metal ball must have reached the starting point
a few milliseconds after the instant t=4.0 s.
Hence, its highest position will have occurred
at the instant of about t=2.0 s
at a height of a little more than y=20 m.
A more precise value for the time at which the ball
reached the starting point, is given at the next page.
We obtain there a value of t=4.08 s.
Using the formula
g=2v0/t,
we find for the acceleration of gravity the value
g=9.8 m/s2.