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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 l.jpg

12 Overview

  • density

  • elasticity & Hooke’s Law

  • Homework:

  • RQs: 8, 9, 10, 11.


Density l.jpg

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


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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

  • 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 air-drag as size increases, causing terminal velocity to increase.


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