Sec. 1.1-1.9
1
Chapter 1
Sections 1.1-1.9
Fluid Mechanics
EM 3313

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Sec. 1.1-1.9
2
What’s the Point?
• Context
– Fluids have many characteristics that make them different
from solids. That’s why we are in this class.
• Motivation
– We are unfamiliar with many important characteristics of
fluids.
•
Learning objective
– Demonstrate knowledge of the properties of fluids
through the mastery of definitions and discussions of their
effects

Sec. 1.1-1.9
3
1.1 What is a fluid?
•
Definition: Stress – force per unit area
–
Normal stress acts perpendicular to a face
–
Shear stress acts parallel to a face
•
From our perspective, there are two states of matter
–
A fluid
is a substance that deforms continuously (flows) under the
application of a shear stress,
no matter how small
•
A fluid at rest must have zero shear stress (hydrostatic). Only a normal
stress
σ
=-p
–
A solid
does not deform continuously (flow) for an arbitrarily small
shear stress
•
Two classes of fluids
–
A liquid
has closely packed molecules with strong cohesive forces
(retains volume and forms a free surface)
–
A gas
has has widely spaced molecules with weak cohesive forces
(no definite volume)

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