An initially-stationary object which is allowed to fall freely under gravity drops a distance which is proportional to the square of the elapsed time. This image, spanning half a second, was captured with a stroboscopic flash at 20 flashes per second.  During the first 1/20th of a second the ball drops one unit of distance (here, a unit is about 12 mm); by 2/20ths it has dropped at total of 4 units; by 3/20ths, 9 units and so on.<br>
To take the picture, the ball - about the size of a tennis ball - was suspended by a short length of black thread and was released as the shutter was opened and the flash triggered. The shutter remained open for the whole of the half-second period, during which time the flash fired multiple times to capture the ball at 1/20 second intervals.
<a href="//commons.wikimedia.org/wiki/User:MichaelMaggs" title="User:MichaelMaggs">MichaelMaggs</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>