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AP Physics Day 3
9-6 Elasticity; Stress and Strain
•Most objects change shape under force or stress.
•If objects are stressed enough, they will fracture.
•Hooke’s Law F=kDL …where DL = change in length.
•See page 253 chart of F –v- DL .
•Notice the terms Elastic region, proportional limit ,
Plastic limit, Elastic limit and Breaking point.
9-6 Continued
•The maximum force that can be applied
without breaking, is called the ultimate strength.
•DL =(1/E)(F/A) L0 Where E is known as the
elastic modulus or Young’s modulus. (p254)
•See Example 9-12 p255.
9-6 continued
•When a rod or material is being stretched by a
force on both ends, it is under tension or tensile
stress.
•When a material or rod has compressive force
on both ends, we say it is under compressive
stress.
•When a lateral force acts on a rod, we say it is
under shear stress.
•For shear strain, use… DL =(1/G)(F/A) L0
•Where G is the shear modulus
9-6 Continued
•If an object is subjected to pressure or force
from all directions, DV/V0 =-(1/B)(DP)
•Where B is the bulk modulus.
9-7 Fracture
•If forces are too much, an object will break, or
fracture.
•See chart on p 258.
•See Example 9-13.
•Pre stressed concrete has stress placed on the
reinforcement during curing of the concrete.
9-8
Spanning Spaces: Arches and
Domes
•The post and beam was the original form of
doorway, the series of arches followed. See p
261
•3-dimensionally, the arch gave rise to the
dome.
•See Example 9-15
HW
P270 44-47, 55, 62