So you're out on the lake in your dinghy, or lost on the
Bonneville Salt Flats in your go cart, and you spy a dot
on the horizon. You know it's not a piece of dirt on your
spectacles; you're getting an ominous feeling that it's a
cigarette boat or a rocket-propelled car that could slice
your pedestrian craft in half.
Should you be worried? Assuming both of you
are traveling at constant velocity, here's how to tell. Take
a bearing on the approaching vehicle; wait a bit (how long
depends on how close it is), and take another bearing. If
the two bearings are the same, you've got a problem.
An easy way to do this, without resorting to a compass or other
technological crutch, is to simply look at it for a while and
see if you have to move your head (or eyes) to keep it in the same
place in your field of vision.
The rule of thumb, therefore, is: Constant bearing implies collision
There are two self-evident exceptions to the rule:
- If you're motionless relative to the other (i.e., your velocities are the
same, or you're both stationary (a special case of a special case))
- If the paths are colinear and the separation between the vehicles is increasing