Figure 11-3 shows how pressure is defined. If you place a small, thin, flat plate in a fluid—a gas or a liquid—you’ll find that the fluid exerts a force on either face of the plate. You can feel these forces if you stick your hand into a swimming pool or a sink full of water. (The same kind of forces act on your hand when it’s surrounded by air, but are very feeble compared to the forces exerted by water.) The net force that the fluid exerts on either face of the plate acts perpendicular to the face. The \(\textbf{pressure}\) of the fluid acting on the face is the magnitude of this perpendicular force divided by the area of the face: