The graphs below show you how the velocity changes with time for a number of different examples.

The horizontal
straight line shows something that is moving with a constant velocity.

Straight lines
slanting upwards show objects whose velocity is increasing at a steady rate – they have
constant positive acceleration. Straight lines slanting downwards show objects whose
velocity is decreasing at a steady rate – they have a constant negative acceleration
(retardation). The steeper the line the greater the acceleration or retardation.

A
curved line shows an object whose acceleration is changing as time goes by.

We
can use these graphs to work out the acceleration of an object.

and so the gradient of the line (change in velocity/time) gives us the acceleration

This is easy to do for a straight-line graph but for the curved line the acceleration is constantly changing and so we must measure the change in velocity over a small time interval to get an accurate answer.