A ladder 10 meters long is leaning against a vertical wall with its
other end on the ground. The top end of the ladder is sliding
down the wall. When the top end is 6 meters from the ground is
sliding at 2m/sec. How fast is the bottom moving away from the
wall at this instant?
Mathematically, the ladder problem is
almost identical to the circle problem that we did earlier. The extra
feature is that it's a story problem. We have to draw a picture and
think about the variables to see that $x^2 + y^2$ is a constant.