Fluid Mechanics Basic Definitions

• Mechanics : Deals with action of forces on bodies at rest or in motion.

• Velocity and Speed: Rate of displacement is called velocity and Rate and distance traveled is called Speed.

Unit: m/s

• Acceleration: Rate of change of velocity is called acceleration. Negative acceleration is called retardation.

• Momentum: The capacity of a body to impart motion to other bodies is called momentum.

The momentum of a moving body is measured by the product of mass and velocity the moving body

Momentum = Mass x Velocity

Unit: Kg m/s

• Newton’s first law of motion: Every body continues to be in its state of rest or uniform motion unless compelled by an external agency.

• Inertia: It is the inherent property the body to retain its state of rest or uniform motion.

• Force: It is an external agency which overcomes or tends to overcome the inertia of a body.

• Newton’s second law of motion: The rate of change of momentum of a body is directly proportional to the magnitudes of the applied force and takes place in the direction of the applied force.

• Mass: Measure of amount of matter contained by the body it is a scale of quantity.

Unit: Kg.

• Weight: Gravitational force on the body. It is a vector quantity.

F = ma

W = mg

Unit: newton (N) g = 9.81 m/s2

• Volume: Measure of space occupied by the body.

Unit: m3

1 m3 = 1000 litres

• Work: Work done = Force x Displacement  Linear motion.

Work done = Torques x Angular displacement  Rotatory motion.

Unit: Nm or J

• Energy: Capacity of doing work is called energy.

Unit: Nm or J

Potential energy = mgh

Kinetic energy = ½ mv2 or ½ mrw2

Power: Rate of doing work is called Power.

• Matter: Anything which possess mass and requires space to occupy is called matter.

• States of matter:

Matter can exist in the following states

• Solid state.

• Fluid state.

• Solid state: In case of solids intermolecular force is very large and hence molecules are not free to move. Solids exhibit definite shape and volume. Solids undergo certain amount of deformation and then attain state of equilibrium when subjected to tensile, compressive and shear forces.

• Fluid State: Liquids and gases together are called fluids. Incase of liquids Intermolecular force is comparatively small. Therefore liquids exhibit definite volume. But they assume the shape of the container Liquids offer very little resistance against tensile force. Liquids offer maximum resistance against compressive forces. Therefore, liquids are also called incompressible fluids. Liquids undergo continuous or prolonged angular deformation or shear strain when subjected to tangential force or shear force. This property of the liquid is called flow of liquid. Any substance which exhibits the property of flow is called fluid. Therefore liquids are considered as fluids.

In case of gases intermolecular force is very small. Therefore the molecules are free to move along any direction. Therefore gases will occupy or assume the shape as well as the volume of the container.

Gases offer little resistance against compressive forces. Therefore gases are called compressible fluids. When subjected to shear force gases undergo continuous or prolonged angular deformation or shear strain. This property of gas is called flow of gases. Any substance which exhibits the property of flow is called fluid. Therefore gases are considered as fluids.