Phy 2053 Announcements
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Test 2 had mean of 14.3, =3.17
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Grades are posted on e-learning
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Solutions are posted on exams page
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See me before Thursday if you want to
challenge computer grading
Make-up exam for students missing exams
1 or 2 will be held in NPB 1101 4/21 from
6:15 pm to 8:10 pm.
http://www.photoshopsupport.com/photoshop-blog
/05/10/11-liquid-sculpture.html
http://p25ext.lanl.gov/~hubert/aerogel/
Chapter 9
Solids and Fluids
www.pbs.org/wgbh/nova/volcano/anat_02.html
States of Matter
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Solid
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Liquid
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Atoms close together
Fixed positions
Electrostatic forces
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Quantum mechanics!
No long range order
(usually) at higher temperatures
Can flow
Stay in open container
Gas
Molecules are in constant random motion
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Molecules exert weak forces on each other
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Average separation is large
compared to the size of the molecules
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Plasma
Gas heated to a very
high temperature
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Many of the electrons
are freed from the nucleus
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I - crystalline
II - amorphous
States of matter – what we
don’t know
What the universe is made of:
?
We know about this
edelweiss.in2p3.fr/Presentation/index.php
?
Strength of materials
load
extension
Reminder: spring force
Robert Hooke
1600s
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F  kx
force is linearly proportional to
displacement
k – spring constant:
- large k: hard spring
- small k: weak spring
historical interlude
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Hooke(1675): “The true Theory of Elaſticity or
Springineſs, and a particular Explication
thereof in ſeveral Subjects in which it is to be
found: And the way of computing the velocity
of Bodies moved by them. ceiiinosssttuu”
ceiiinosssttuu  “ut tensio sic vis”
“As extension, so is force”
(Hooke wanted to apply theory to
design of clocks, to establish scientific
priority, but didn’t want to tell the
competition!)
Elastic Properties of Solids
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Stress is the force per unit area
causing the deformation
Strain is a measure of the amount
of deformation
The elastic modulus is the constant
of proportionality between stress
and strain
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For sufficiently small stresses, the stress
is directly proportional to the strain
The constant of proportionality depends
on the material being deformed and the
nature of the deformation
like Hooke!
Quantitative characterization of mechanical
properties of materials
Elastic behavior:
Hooke’s Law
Young’s Modulus:
Elasticity in Length
A
L0
L
F
Stress = F/A
Strain = L/L0
Young’s Modulus:
F
L
Y
A
Lo
Stress=Y · strain
A material which has a high Young’s modulus
is harder to stretch (or compress)
Why do we use force per unit area
(F/A) in the formula ??
Note
 YA 
F
L
Y
F 
 L
A
Lo
 Lo 
like Hooke’s law, but (…) is not characteristic of material
e.g., if wire is 2x as thick, (…) is 4x bigger!
http://www.youtube.com/watch?v=3UZuPayyAnM
Example: strength of steel rod
For steel, Y=2.0x1011 N/m2
Elastic limit: 2.5x108 N/m2
Breaking point: 24.0x108 N/m2
What is force required to reach
the elastic limit for 20 cm long
1cm x 1cm square steel rod?
How far is bar stretched at
this force?
Similar concepts: bulk and shear modulus
Y