What Is Flotation Concept ?

 

Flotation concept     INTRODUCTION   The branch of physics which deals principally with the properties of liquids is called hydrostatics. It includes consideration of the equilibrium of liquids as well as of bodies in equilibrium under the action of forces, some of which are produced by liquids.   The fundamental laws on which hydrostatics is founded are only two in number :   (i) The force exerted by a liquid on any surface with which it is in contact, is perpendicular to that surface and is called 'thrust'.     (ii) Any pressure communicated to a liquid is equally transmitted through the whole liquid in every direction. These two laws are understood with the help of concepts of thrust and pressure. Hence, we shall begin this chapter by defining these two key terms.   THRUST AND PRESSURE   Thrust. A body always exerts a force equal to its weight on any surface supporting it. The effect of this force on the surface depends on the area on which it is spread. Thus,   Force acting normally on a surface is called the thrust.   Thrust is a vector quantity and is measured in the unit of force, i.e., newton (N). Pressure. In order to understand the concept of pressure, let us consider some examples related to practical life.   1. When you stand on loose sand, your feet sink deeply into it. But if you lie down on it, your body sinks in very little. In either case, the force exerted on the sand is due the weight of your body and is the same. When you stand, this force is acting on a smaller area of the sand but when you down, the same force acts on a larger area of the sand.   2.A sharp knife cuts easily than a blunt knife by applying the same force.   3. A sharp needle pressed against our skin pierces it. But a blunt object with a wider contact area does not affect the skin when pressed against it with the same force.   4. If an elephant happens to step on a man's chest, his ribs would crack. But a circus performer saves himself from such an accident by placing a large, light but strong wooden plank on his chest.   From the above discussion, it follows that the effect of the forces of the same magnitude on different areas is different. Smaller the area on which the force acts, greater is the impact. Thus, in considering the action of a force on a surface, we must think of force acting on a unit area rather than the total force. This leads us to the concept of pressure.   The thrust acting on unit area of the surface is called the pressure.   P=F/A   A Units of Pressure. The SI unit of pressure is called pascal (Pa) in honour of Blaise Pascal.   1 Pa = 1 N/m2   One pascal is defined as the pressure exerted on a surface of area of 1 m2 by a thrust of 1 N (acting normally on it).   Other units of pressure are bar and millibar (m bar) where 1 bar = 105 N/m2 and 1 millibar = 102 N/m2   It is a common practice in meteorology to measure atmospheric pressure in bars and millibars. Further, 1 atmospheric pressure (1 atm) = 101.3 k Pa = 1.013 bar = 1013 m bar   SOME INTERESTING ASPECTS OF PRESSURE   Since pressure is dependent on the surface area in contact, it is, therefore, important to know the area over which a given force acts. This fact is made use of in several practical situations, some of which are briefly discussed below.   1. The foundation of a building or a dam has a large surface area so that the pressure exerted by it on the ground is less. This is done to prevent the sinking of the building or the dam into the ground.   2. The tyres of a bus or a truck have larger width than those of a car. Further, the number of tyres of heavy vehicles is more than four. This is done to enable the tyres to carry more weight and to prevent sinking into ground.   3. Cutting tools are either made sharp or pointed in order to produce maximum pressure with minimum thrust.   4. Nails and pins have pointed ends so that these can be fixed with minimum force because the pressure on the pointed ends would be large.   5. A sleeping mattress is so designed that when you lie on it, a large area of your body comes in its contact. This reduces the pressure on the body and sleeping becomes comfortable.   6. Wide wooden sleepers are kept below railway lines to reduce pressure on the railway tracks and prevent them from sinking into ground.   On Your Tips   1. Force acting normally on a surface is called thrust.   2. Pressure (P) is the thrust (F) per unit area of the surface, i.e., P = F/A. Tips 3. The same force acting on a smaller area exerts a larger pressure and a smaller pressure while acting on a larger area.   4. The unit of pressure is pascal (Pa) where 1 Pa = 1 N/m2. Other units of pressure are (i) bar (ii) millibar (m bar) and (iii) atm 1 bar = 105 Pa and 1 m bar = 102 Pa 1 atmospheric pressure (1 atm) = 1.013 105 N/m2 = 1.013 105 Pa   =101.3 k Pa = 1.013 bar     NOTE   It is surprising to think that how a human being can withstand such an enormous thrust (15.2 ton wt) without any discomfort while it is impossible for him even to carry a load of one ton. There are openings and innumerable pores on the skin of the body through which air goes within the system. As such there is free communication between inside and outside of the body. The pressure of air inside the body counter-balances the pressure outside the body. Consequently, just as a lump of cotton in air can fly about safely without being squeezed by atmospheric pressure, any delicate part of the human system is equally safe. The pressure of blood within veins is matched against the outside atmospheric pressure. But if the outside atmospheric pressure alters, for example, in climbing up high altitude or in living under sea, precautions should be taken to compensate the difference of pressures inside and outside the body.