Transcript Ch.12 Solids

12 Overview
• density
• elasticity & Hooke’s Law
• Homework:
• RQs: 8, 9, 10, 11.
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Density
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Density = mass/volume
depends on chemical type and structure
water 1.0 grams/cm3
ice 0.9 grams/cm3
copper 8.9 grams/cm3
lead 11.9 grams/cm3
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Density Units
• 1.0 grams/cm3 = 1,000 kg/m3
• think of 264 gallons of water, each gallon
weighs 8.35 lbs.
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Elasticity - ability to regain shape
• elastic material – perfectly regains shape
• inelastic material – incompletely regains
shape
• elastic limit – force in which an elastic
material becomes inelastic
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Hooke’s Law
• Elastic Force ~ amount of stretch or
compression
• F ~ Dx; F/Dx = spring constant (stiffness)
• Example: many springs obey Hooke’s Law
within their elastic limit.
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Longer objects are
easier to stretch.
Thicker objects
are harder to
stretch.
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Summary
• density is mass/volume, 1.0 grams/cm3 =
1,000 kg/m3
• elastic materials return to original shape
• Hooke’s Law: deformation ~ force
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Crystal Structure
solids with a regularly repeating pattern are crystals
Examples: Graphite
NaCl
hexagonal
cubic
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X-Ray
Crystallography
X-Rays are used to
determine the structure of
crystalline solids
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raindrops
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which falls faster, small or large raindrops?
5 mm drops: ~9 m/s (20 mph)
0.5 mm: ~2 m/s (4 mph)
why?
A. Scaling
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Scaling
• mass & weight increase at L3 , but area &
strength increase with L2.
• raindrops: weight increases faster than airdrag as size increases, causing terminal
velocity to increase.
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