The various quantities used in fluid mechanics may be subdivided into the categories of *scalars*, *vectors*, and *tensors*. At any point in space and time, a *scalar* needs only a single number to represent it, examples being temperature, volume, and density. A *vector*, however, needs for its description both a *magnitude* and a *direction*, examples being force, velocity, and momentum; for example, the gravitational force on one kilogram is *g* newtons, *vertically downward*. A *tensor* is more complicated, and its discussion will be postponed until the shear-stress tensor is introduced in Section 5.7. Appendix C gives details of a few important vector and tensor operations ...

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