One day a Zebra danio named Danny is at rest at (0,0) when he notices a hungry shark swimming directly towards him along the positive x axis.
The visual angle is 0.6 radians and increasing at 0.06 radians/sec! Immediately Danny turns and swims as fast as he can, directly away from the shark. But looking back he sees that the visual angle is still increasing, though at a slower rate of 0.04 radians/sec. Obviously the shark is faster than Danny, and will soon overtake him. Evasive action is needed! Danny hopes that the shark will keep moving in the same direction, so all he needs to do is get out of the way. If he can get to y = S/2 before the shark catches up to him, he will be safe. But in which direction should he swim?
Fortunately, Danny has not wasted all that time he has spent in schools: he has taken Math 102 and can do calculus problems instantly in his head, as well as measuring visual angles and their rates of change.
