Lecture 1
Mechanics: The oldest physical science that deals with both stationary and
moving bodies under the influence of forces.
Statics: The branch of mechanics that deals with bodies at rest.
Dynamics: The branch that deals with bodies in motion.
Fluid mechanics: The science that deals with the behavior of fluids at rest
(fluid statics) or in motion (fluid dynamics), and the interaction of fluids with solids or other fluids at the boundary.
Fluid dynamics: Fluid mechanics is also referred to as fluid dynamics by considering fluids at rest as a special case of motion with zero velocity.
Hydrodynamics: The study of the motion of fluids that can be approximated as incompressible (such as liquids, especially water, and gases at low speeds).
Hydraulics: A subcategory of hydrodynamics, which deals with liquid flows in pipes and open channels.
Gas dynamics: Deals with the flow of fluids that undergo significant density changes, such as the flow of gases through nozzles at high speeds.
Aerodynamics: Deals with the flow of gases (especially air) over
bodies such as aircraft, rockets, and automobiles at high or low speeds. Meteorology, oceanography, and
hydrology: Deal with naturally occurring flows.
Difference between solid,
Liquid and Gas
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✓The solid state is one of the fundamentals tates of matter :
Solids differ from liquids and gases by the characteristic of rigidity.
The molecules of solids are tightly packed because of strong intermolecular forces; they only oscillate about their mean position
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Solids can be defined as the state of matter which has definite shape and volume and has a rigid structure.
Solids possess the least compressibility
and thermal expansion.
Example: Iron (Fe)
The molecules in a liquid are closely packed due to
weak intermolecular forces.
Liquids
These forces are weaker than solids but stronger than that of gases.
There is much space in between the molecules of liquids which makes their flowing ability easy.
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Liquids can easily acquire the shape of a vessel, and they have a fixed volume.
Conversion of solids into liquids takes place when we increase the temperature of solids to a point where solids begin to melt.
Generally, the density of liquid lies between the density of solids and gases. Compressibility and thermal expansion of liquids are slightly higher than that of solids.
Example: Water (H2O)
In this state of matter, distances between the Gases molecules are large (intermolecular distance is in the range of 10^−7 – 10^−5cm).
The intermolecular forces experienced between them are negligible.
Thus, translatory, rotatory and vibratory motions are observed prominently in gases.
Gases do not have any fixed shape or volume.
They also possess high compressibility and thermal expansion.
Example: Oxygen (O2)
What is a Fluid?
Fluids is a substance that cannot resist a shear force or stress without moving as can a solid.
It is usually classified as liquids or gases.
A liquid has intermolecular forces that hold it together so that it possesses volume but no definite shape.
A fluid deforms continuously under the influence of a shear stress, no matter how small.
In solids, stress is proportional to strain, but in fluids, stress is proportional to strain rate.
When a constant shear force is applied, a solid eventually stops deforming at some fixed strain angle, whereas a fluid never stops deforming and approaches a constant rate of strain.
Definition: Fluid
Fluid: Fluid is a substance that deforms continuously under the action of shear stress, as long as the shear stress is applied, no matter how small the shear stress may be.
It is this property of fluids not to resist shear stress that makes them capable to flow and take any shap
Definition: Liquid vs Gasses
In a liquid, molecules can move relative to each other, but the volume remains relatively constant because of the strong cohesive forces between the molecules.
As a result, a liquid takes the shape of the container it is in, and it forms a free surface in a larger container in a gravitational field.
A gas, on the other hand, expands until it encounters the walls of the container and fills the entire available space.
This is because the gas molecules are widely spaced, and the cohesive forces between them are very small.
Unlike liquids, gases cannot form a free surface.
Differences
Fluids
• Fluids have no shape
• Fluids cannot sustain a shear force, i.e. a fluid is always in motion • Stress is a function of the rate of strain, thus a fluid had a dynamic' state
• The static properties of a fluid cannot be extended to dynamic properties
Solid
Solids have a definite shape
• Solids can sustain a shear force; i.e. they remain static
• Stress is a function of strain, thus a solid maintains a static or `quasi-static' state.
• The static properties of a solid can be extended to dynamic properties.
Similarities
The continuum hypothesis is used for both fluids and solids.
The fundamental laws of mechanics apply to both fluids and solids.
- Newton's law of motion (conservation of momentum) - Conservation of Mass
- First law of thermodynamics (conservation of energy)
The constitutive law relating stress and rate of strain also apply to both.
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