### Theory:

Airplanes, aircraft and ships not only maintain their speed but also their direction. Take ship, for an example, it has to maintain its speed and direction. And that's where average velocity concepts comes into play. Similar to the average speed we can also define average velocity.

Average velocity:
Average velocity can be defined as the ratio between the total displacement and the total time taken.

Zero Displacement:

In some circumstances the displacement can be zero. Let's see an example to understand this clearly.

Consider that you're a formula one race car driver, you finished a 250 km race in one hour and thirty minutes. You finished at the place where you started.

Then what will be your average velocity?

We know that average velocity is .

First we have to find the displacement. But the important thing is you starting and finishing points are the same. Therefore the displacement will be zero.

If the displacement is zero then the average velocity is also zero.
Important!
Note: If the initial and final point of an object is in the same quantity then the displacement is zero. Because there is no displacement occurred.
Relation between $$v$$, $$d$$ and $$t$$.

The below triangle diagram can help you to recall the relationship between velocity ($$v$$), displacement ($$d$$), and time($$t$$). The relation is,

$\begin{array}{l}\mathit{Velocity}\phantom{\rule{0.147em}{0ex}}\left(v\right)=\frac{\mathit{Displacement}\phantom{\rule{0.147em}{0ex}}\left(d\right)}{\mathit{Time}\phantom{\rule{0.147em}{0ex}}\left(t\right)};\\ \\ \mathit{Time}\phantom{\rule{0.147em}{0ex}}\left(t\right)=\frac{\mathit{Displacement}\phantom{\rule{0.147em}{0ex}}\left(d\right)}{\mathit{Velocity}\phantom{\rule{0.147em}{0ex}}\left(t\right)}\\ \\ \mathit{Displacement}\phantom{\rule{0.147em}{0ex}}\left(d\right)=\mathit{Velocity}\phantom{\rule{0.147em}{0ex}}\left(t\right)\phantom{\rule{0.147em}{0ex}}×\mathit{Time}\phantom{\rule{0.147em}{0ex}}\left(t\right)\end{array}$

Therefore simply we can write, $v=\frac{d}{t};t=\frac{d}{v};d=\mathit{vt}$.