This chapter introduces the description of fluid motion, that is, the fluid kinematics. At first, the Lagrangian and Eulerian method is compared to emphasize that most problems in fluid mechanics is more suitable for Eulerian method. Secondly, the concepts of pathlines and streamlines are introduced. Next, Acceleration equation and substantial derivative are derived in Eulerian coordinates and their physical significance is discussed in depth and in examples. Reynolds transport theorem is then introduced and compared with substantial derivative to demonstrate that they are the same relation in integral and differential form respectively. Deformation of a finite fluid element is discussed in the next. Linear deformation, rotation, angular deformation equations are derived individually with equations and illustrations. These knowledges are the key to derive the differential equations of a flow, which will be introduced in chapter 4.

This chapter discusses two different approaches to describing fluid flow: a Lagrangian approach (following a fluid element as it moves) and a Eulerian approach (watching fluid pass through a fixed volume in space). Understanding each of these descriptions of flow is needed to fully understand the dynamics of fluids. This chapter is devoted to diving into the differences between the two descriptions of fluid motion. Understanding this chapter will help tremendously in the understanding of the upcoming chapters when the Navier–Stokes equations and energy equation are discussed. This chapter will introduce the material derivative. It is extremely important to understand this derivative before the Navier–Stokes equations themselves are tackled.