If you drop a ball or stone, it falls to the ground. Figure 2.21, based on a multiflash photograph, shows the ball at equal intervals of time. You can see that the ball’s velocity increases as it falls because the spaces between the images of the ball increase steadily. The ball is accelerating.
A multiflash photograph is useful to demonstrate that the ball accelerates as it falls. Usually, objects fall too quickly for our eyes to be able to observe them speeding up. It is easy to imagine that the ball moves quickly as soon as you let it go, and falls at a steady speed to the ground. Figure 2.21 shows that this is
not the case.
If we measure the acceleration of a freely falling object on the surface of the Earth, we find a value of about $9.81\,m\,{s^{ - 2}}$. This is known as the acceleration of free fall, and is given the symbol g:

acceleration of free fall $g = 9.81\,m\,{s^{ - 2}}$

The value of g depends on where you are on the Earth’s surface, but we usually take $g = 9.81\,m\,{s^{ - 2}}$.
If we drop an object, its initial velocity $u = 0$. How far will it fall in time t? Substituting in gives displacement s:

$\begin{array}{l}
s = \frac{1}{2} \times 9.81 \times {t^2}\\
 = 4.9 \times {t^2}
\end{array}$

Hence, by timing a falling object, we can determine g.