ELASTICITYPhysics Grade XI2016

Aim of this lessonExplain the effect of the force to the elasticity character and vibration

Sub Chaptera. Elasticityb. Simple Harmonic Motion

Elasticity• the ability of an object or material to resume its normal

shape after being stretched or compressed

Inelasticity ?

Stress, Strain and Modulus of Elasticity

Stress• Stress is "force per unit area" - the ratio of applied force F

to cross section area - defined as "force per area".

• tensile stress - stress that tends to stretch or lengthen the material - acts normal to the stressed area

• compressive stress - stress that tends to compress or shorten the material - acts normal to the stressed area

• shearing stress - stress that tends to shear the material - acts in plane to the stressed area at right-angles to compressive or tensile stress

• Tensile or compressive stress normal to the plane is usually denoted "normal stress" or "direct stress" and can be expressed as

• σ = Fn / A         (1)• where• σ = normal stress ((Pa) N/m2)• Fn = normal component force (N)• A = area (m2)

Example• A force of 10 kN is acting on a circular rod with diameter

10 mm. The stress in the rod can be calculated as• σ = (10 x103 N) / (π ((10 x10-3 m) / 2)2)•    = 127388535 (N/m2) •    = 127 (MPa)

Strain• Strain is defined as "deformation of a solid due to stress"

and can be expressed as         

whereΔL = change of length (m)L = initial length (m)ε = strain - unitless

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Modulus of ElasticityE = Young's modulus (Modulus of Elasticity) (N/m2 (Pa)

Young's modulus can be used to predict the elongation or compression of an object.

• Elasticity Constant

Hooke’s LawHooke's law is a principle of physics that states that the force F needed to extend or compress a spring by some distance X is proportional to that distance.

where k is a constant factor characteristic of the spring X is small compared to the total possible deformation of the spring.

Example

Calculate the value of spring constant !

F = k Δx40 N = k (0.08 m)K = 500 N/m

SWTextbook “Exercise”Page 96Page 98Page 100

Simple Harmonic Motion

𝒂=𝒌𝑨𝒎 𝐜𝐨𝐬 (𝝎𝒕+𝜽𝟎)

𝑇=2𝜋 √ 𝑙𝑔

Hooke’s Law for the composition

Series

Parallel

Give us SW please....