You are exerting a force on the book; the book is feeling this force by virtue of your hands being in contact with it. Similarly, a body immersed in a liquid or a gas (a fluid) feels a force by virtue of the body surface being in contact with the fluid. The forces exerted by the fluid on the body surface derive from two sources. One is the pressure exerted by the fluid on every exposed point of the body surface. The net force on the object due to pressure is the integrated effect of the pressure summed over the complete body surface. In the aerodynamic flow over a body, the pressure exerted by the fluid is different at different points on the body surface (i.e., there is a distribution of variable values of pressure over the surface). At each point, the pressure acts locally perpendicular to the surface. The integrated effect of this pressure distribution over the surface is a net force—the net aerodynamic force on the body due to pressure. The second source is that due to friction between the body surface and the layer of fluid just adjacent to the surface.
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