This Problem Set is a part of the Lesson 1, Unit 5, Grade 8. Students realize that an assumption of a constant rate of motion is not appropriate for all situations. Even if this is not possible, we may be able to use data from the situation to see a relationship of some kind between the values of one quantity and their matching values of the second quantity. Mathematicians call any clearly-described rule that assigns to each value of one quantity a single value of a second quantity a function.