Gravitation

THRUST AND PRESSURE:

A liquid exerts pressure.To show it,consider a vessel containing a liquid.If it has a small hole near the bottom,the liquid will flow out of it.If the motion of the liquid is stopped by pressing a finger against the hole,the finger will experience an outward force  due to the liquid contained in the vessek.Therefore,it can be concluded that the liquid exerts pressure on the walls and base of the containing vessel

Thrust.The total force exerted by a liquid on any surface in contact with it ,is called thrust of liquid
Pressure.The thrust exerted by a liquid (at rest) per unit area of the surface in contact with the liquid is called pressure.
If F is the thrust exerted by a liquid on a surface of area A,then pressure is given by
            \( p = \frac{F} {A} \) 
The unit of pressure is dyne cm^-2 in cgs system and Nm^-2 in SI.
A pressure of 1Nm^-2  is also called1 pascal (Pa)
        1Pa (or 1Nm^-2)=10 dyn Cm^-2
The dimensional formula of pressure is \( [ML^{ - 1} T^{ - 2} ] \) 

Atmospheric pressure (P_atm):
Atmospheric pressure is the pressure exerted by environmental air. (or)  The force exerted by atmosphere per unit area on the surface of earth is called as  Atmospheric pressure.
The magnitude of average atmospheric pressure on the surface of earth is 76 cm of Hg.
The instrument used for measuring of pressure is called Barometer. It was constructed by Torricelli in 1643.
Other kinds of barometers are i) Aneroid barometer    ii) Fortin’s barometer.
Atmospheric pressure decreases with the increase of height (or) altitude.

Gauge Pressure (P_gauge):
The pressure that is measured with respect to atmospheric pressure is called gauge pressure.
Gauge Pressure can be positive (+ve) or Negative (-ve).
Vacuum pressure (P_Vacuum) or pressures less than atmospheric are known as negative (-ve) gauge pressure.

Absolute pressure (P_abs):
It is the pressure that is measured with respect to the zero pressure line. Absolute pressure is always positive.
Note: The excess pressure above atmospheric pressure is usually called gauge 
pressure and the total pressure is called absolute pressure. Thus, 
            Gauge pressure = absolute pressure – atmospheric pressure.
When a fluid (either liquid or gas) is at rest, it exerts a force perpendicular to any surface in contact with it, such as a container wall or a body immersed in the fluid.
While the fluid as a whole is at rest, the molecules that makes up the fluid are in motion, the force exerted by the fluid is due to molecules colliding with their  surroundings.
If we think of an imaginary surface within the fluid, the fluid on the two sides of the surface exerts equal and opposite forces on the surface, otherwise the surface would accelerate and the fluid would not remain at rest. 
                         
Consider a small surface of area dA centered on a point on the fluid, the normal force exerted by the fluid on each side is dF_^ . The pressure P is defined at that point as the normal force per unit area, i.e. \( P = \frac{{dF_ \bot }} {{dA}} \) ,  
If the pressure is the same at all points of a finite plane surface with area A, then \( P = \frac{{F_ \bot }} {A} \)  
Where  is the normal force on one side of the surface. The SI unit of pressure is pascal, where 
    1 pascal = 1 Pa = 1.0 N/m²
 One unit used principally in meterology is the Bar which is equal to 10⁵ Pa. 
    1 Bar = 10⁵ Pa.