12 Overview density elasticity Hookes Law Homework RQs

12 Overview • density • elasticity & Hooke’s Law • Homework: • RQs: 8, 9, 10, 11. 1

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 2

Density Units • 1. 0 grams/cm 3 = 1, 000 kg/m 3 • think of 264 gallons of water, each gallon weighs 8. 35 lbs. 3

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 4

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. 5

Longer objects are easier to stretch. Thicker objects are harder to stretch. 6

Summary • density is mass/volume, 1. 0 grams/cm 3 = 1, 000 kg/m 3 • elastic materials return to original shape • Hooke’s Law: deformation ~ force 7

Crystal Structure solids with a regularly repeating pattern are crystals Examples: Graphite Na. Cl hexagonal cubic 8

X-Ray Crystallography X-Rays are used to determine the structure of crystalline solids 9

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 10

Scaling • mass & weight increase at L 3 , but area & strength increase with L 2. • raindrops: weight increases faster than airdrag as size increases, causing terminal velocity to increase. 11