The diagram shows a velocity-time graph for an object falling through a fluid, eg air, water, oil. Between A and B. The object accelerates at first because of the force of gravity.

Diagrams of Mean Velocity of Uniform Motion of Water in open Channels, based on the Formula of Ganguillet and Kutter. By Prof. Irving P. Church. 11 Diagrams + 1 page Text. (New York: Wiley and ...