Runners stand at first and second base in a baseball game. At the moment a ball is hit, the runner at first base runs to second base at 18 ft/s; simultaneously, the runner on second runs to third base at 20 ft/s. How fast is the distance between the runners changing 1 second after the ball is hit (see figure)? The distance between consecutive bases is 90 ft and the bases lie at the corners of a square.

90 ft 2nd base 1st base 3rd base 90 ft Home plate