The airplane above is traveling in a straight line at a constant speed. Its speed is the distance traveled divided by the time interval. If we know how far it is traveling in a given time, we can determine its speed.

We can calculate this speed if we have two snapshots of the plane separated in time as illustrated by the following two images:

The image below was taken two seconds later, after the plane traveled 400 meters. Thus, the airplane speed is 200 meters per second. Notice the distance is measured from the same spot on the plane, in this example from the nose.

In the above example the plane traveled in a straight line during the time we tracked it.

What happens if the object we are tracking does not travel in a straight line?