Fluid Mechanics
* Fluids have greater friction with each other than solids.
* A pure liquid is incompressible and has a free surface of its own. A gas is compressible and it expands to occupy all the space available to it.
* If 'F' is the normal force exerted by a fluid on an area 'A' then the average pressure 'P' is defined as the ratio of the force to area.
P = F
A
* Pascal's law:- Pressure in a fluid at rest is same at all points which are at the same height. A change in pressure applied to an enclosed fluid is transmmited undiminished to every point of the fluid and the walls of the containing vessel.
* The pressure in a fluid varies with depth 'h' according to the expression:
P = Pa + 
gh where is the density of the fluid, assumed uniform and "Pa" is the atmospheric pressure.
* Bernoulli's principle:- As we move along a streamline, the sum of the pressure (P), the kinetic energy (1/2v2) and potential energy per unit volume (gh) remains constant. P + (1/2v2) + (gh) = constant.
* When a shear stress is applied to a fluid, the motion is generated which is called a shear growing with time. The ratio of the shear stress to the time rate of shearing strain is known as coefficient of viscosity, 
.
* Stoke's law states that the viscous drag force 'F' on a sphere of radius 'r' moving with velocity 'v' through a fluid of viscosity is:-
F = -6
rv.
* The onset of turbulence in a fluid is determined by a dimensionless parameter called the Reynolds number given by:-
Re = vd/ where 'd' is a typical geometrical length associated with the fluid flow and the other symbols have their usual meaning.
* Surface tension is a force per unit length acting in the plane of interface between the liquid and the bounding surface. It is the extra energy that the molecules at the interface have as compared to the interior.
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