In this module, we will be reviewing the basic principles of knowledge area ten fluid mechanics. The outline of fluid mechanics in the FE manual is like this. First of all, we do flow, flow measurement. Then fluid properties. Then fluid statics, and then energy impulse and momentum equations. The problem with this, though, is that more commonly we follow this outline, which is what I will do here. First of all, we typically look at fluid properties, and address what is a fluid, then we do fluid statics, forces and pressures in stationary fluids. Then we look at continuity and energy equations. Which are needed for understanding and applying flow measurement. And then we'll finish with similitude and dimensional analysis. And this will be the outline I will follow in this module. So first of all, we will be looking at fluid properties. And in fluid properties, we'll first look at what is a fluid. Then particular thought properties that are important for this segment. Then we'll look at stresses, normal and tangential stresses the normal stresses being pressure, tangential stresses being sheer stresses. Then we'll look at surface tension. And finally, I will conclude with a few comments on properties. And the system of units used. Fluid mechanics is the study of fluids at rest and in motion. And this brings up an obvious question of, what is a fluid? And we can answer that question intuitively, that a fluid is a substance that flows. But a more precise definition is this. A fluid moves continuously under the action of a shear stress, no matter how small. This definition has a number of corollaries. The first corollary is that a fluid cannot support a shear stress. Without moving. Because if there wasn't a sheer stress, it would start flowing. We can illustrate the essential difference between a solid and a fluid by a simple example. Let's suppose I have a block of some solid material here, which is firmly clamped to the table. This could be a block of wood, or a solid, piece of metal, iron, or whatever. And, let's suppose that we apply a force to this solid material on the upper boundary. In other words, a shear force, which results in a shear stress. So, if the solid is clamped firmly to the table. We know what will happen. The solid will deform. And it'll deform in a shape which is something like this. But the important thing about a solid, is that the amount of deformation is finite. It gives a small amount and then stops. And more particularly for solids, we know that the amount of deformation. Is directly proportional to the applied force or the applied stress. But imagine now that this material was a fluid like a blob of water sitting on the tabletop. Well, firstly it couldn't stay there like that of course. If this was a fluid like water, it would immediately start to flow to the sides. Even without a, application of a sheer stress. Now, another corollary is that a fluid at rest has no sheer forces because if it did have sheer forces it wouldn't be at rest. So it follows, then, that if we don't have any sheer forces, we only have normal forces which are pressure forces. Fluids can be subdivided into liquids or gases. And the essential difference is that in liquids, the molecules of the liquid are close packed. They have strong cohesive forces between them. The liquid occupies a definite volume. And it will generally have a free surface. Like this water sitting in this beaker of fluid. It has a definite volume and at the surface here, we have a definite free surface. Gases on the other hand, are characterized by very widely spaced molecules. The spacing between them is very large. The forces between them are negligible and generally speaking, a gas will expand to fill whatever container is, it is enclosed in. Fluids are materials such as air, water, oil. Gasoline, mercury, steam, et cetera are all examples of fluids. But some materials don't fit readily into this classification. For example, asphalt, or lead behave like solids for short periods of time. But if we wait for long times, they begin to flow similarly to a fluid. Other mixtures behave in complex. Manner, for example, slurry mixtures of mud and plastics. And, sometimes, we have to consider fluids which exist in two phases, liquid and gas. For example, a mixture of steam and water, or air bubbles and water. Now fluids are ultimately composed of molecules, which we have said in the case of liquids, are closely packed. Or in gasses, have a large separation between them. But we are generally not interested in microscopic scales in engineering fluid mechanics. We're interested in behavior at larger scales, at macroscopic scales. So although the microscopic scales are important, for example the mass of the fluid is contained within the molecules, we have to have a link between the microscopic and the macroscopic scale. And that link is provided by the continuum hypothesis, which says that any fluid can be divided and subdivided indefinitely into infinitesimally smaller and smaller volumes without ever encountering a sudden discontinuity in properties. In other words, the properties are continuous in space. And, we will employ this and look at the implications of this in the next segment.