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12 Overview density elasticity & Hooke’s Law Homework: RQs: 8, 9, 10, 11. Density Density = mass/volume depends on chemical type and structure water 1.0 grams/cm 3 ice 0.9 grams/cm 3 copper 8.9 grams/cm 3 lead 11.9 grams/cm 3 Density Units 1.0 grams/cm 3 = 1,000 kg/m 3

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## 12 Overview

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**12 Overview**• density • elasticity & Hooke’s Law • Homework: • RQs: 8, 9, 10, 11.**Density**• 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**Density Units**• 1.0 grams/cm3 = 1,000 kg/m3 • think of 264 gallons of water, each gallon weighs 8.35 lbs.**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**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.**Longer objects are easier to stretch. Thicker objects are**harder to stretch.**Summary**• density is mass/volume, 1.0 grams/cm3 = 1,000 kg/m3 • elastic materials return to original shape • Hooke’s Law: deformation ~ force**Crystal Structure**solids with a regularly repeating pattern are crystals Examples: Graphite NaCl hexagonal cubic**X-Ray Crystallography**X-Rays are used to determine the structure of crystalline solids**raindrops**• 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**Scaling**• mass & weight increase at L3 , but area & strength increase with L2. • raindrops: weight increases faster than air-drag as size increases, causing terminal velocity to increase.

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