Benchmark 9B
The Mathematical World: Symbolic Relationships
Grades 9-12, page 220

In some cases, the more of something there is, the more rapidly it may change (as the number of births is proportional to the size of the population). In other cases, the rate of change of something depends on how much there is of something else (as the rate of change of speed is proportional to the amount of force acting).

See the general discussion in Unifying Concepts and Processes: Constancy, change, and measurement.