. Introduction . Pressure . Streamline flow .
Bernoulli’s principle . Viscosity . Surface tension Summary Points to ponder Exercises Additional exercises Appendix chest a large, light but strong wooden plank is placed first, is saved from this accident. Such everyday experiences convince us that both the force and its coverage area are important.
Smaller the area on which the force acts, greater is the impact. This impact is known as pressure. When an object is submerged in a fluid at rest, the fluid exerts a force on its surface. This force is always normal to the object’s surface.
This is so because if there were a component of force parallel to the surface, the object will also exert a force on the fluid parallel to it; as a consequence of Newton’s third law. This force will cause the fluid to flow parallel to the surface. Since the fluid is at rest, this cannot happen. Hence, the force exerted by the fluid at rest has to be perpendicular to the surface in contact with it.
This is shown in Fig. . (a). The normal force exerted by the fluid at a point may be measured.
An idealised form of one such pressure-measuring device is shown in Fig. . (b). It consists of an evacuated chamber with a spring that is calibrated to measure the force acting on the piston.
This device is placed at a point inside the fluid. The inward force exerted by the fluid on the piston is balanced by the outward spring force and is thereby measured. If F is the magnitude of this normal force on the piston of area A then the average pressure P av is defined as the normal force acting per unit area. av = ( .
) In principle, the piston area can be made arbitrarily small. The pressure is then defined in a limiting sense as P = lim ∆ A → ∆ ∆ ( . ) Pressure is a scalar quantity. We remind the reader that it is the component of the force normal to the area under consideration and not the (vector) force that