Fluid dynamics is the study of liquids and gases in motion. It often deals with rates of change over a period of time like velocity. Velocity can be defined as the displacement of a moving body with respect to time. If displacement is represented as x then velocity can be written as
The d in the expression means "change in" so velocity can be described as change in displacement over a change in time.
Similarly acceleration can be described as a change in velocity over a change in time.
When a function has more than one variable, is used instead of d to mean "change in".
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