-Elastic Properties of Solids -Elastic Properties of Solids

AP Physics CMrs. Coyle

• Real objects are not rigid but can deform under the influence of an external force.

• Remember: Hooke’s Law F=-kx

Elastic Properties of SolidsElastic Properties of Solids

• Length (Tensile Stress- Strain)

• Shape (Shear Stress-Strain)

• Volume (Bulk Stress-Strain)

Elasticity of Length- CharacteristicsElasticity of Length- Characteristics

• Tensile Strain= L

L

Tensile Stress=F/A

Elasticity in LengthElasticity in Length

• Tensile Stress = Y x Tensile Strain

• Y is known as Young’s Modulus• Units of Y are N/m2

F LY

A L

Tensile Strength Test

• Tensile Strength Sample

Ex: #30

• A steel wire of diameter 1mm can support a tension of 0.2kN. A cable to support a tension of 20kN should have a diameter of what order of magnitude?

• Ans: 1cm

Elasticity in Shape - Shear Stress= F / A

Example: Shear Stress

http://www.fema.gov/plan/prevent/earthquake/fema74/chapter2_2.shtm

http://www.fema.gov/plan/prevent/earthquake/fema74/chapter2_2.shtm

Shear Stress and Shear StrainShear Stress and Shear Strain

F xS

A h

S is the Shear Modulus,

Unit : N/m2

Ex: #33

If the shear stress in steel exceeds 4.00x108 N/m2

the steel ruptures.

a)Determine the shearing force necessary to shear a steel bolt 1.00cm in diameter.

Ans: a) 3.14x104 N

Ex: #33 cont’d

b)Determine the shearing force necessary punch a 1.00cm diameter hole in a steel plate 0.500cm thick.

Ans: b) 6.28x104 N

Elasticity in VolumeElasticity in Volume

F VB

A V

B is the Bulk Modulus

Unit: N/m2

• Note that ΔF/A is pressure.

Ex:#35

When water freezes, it expands by about 9%. What pressure increase would occur inside

your automobile engine block if the water in it froze?

(The bulk modulus of ice is 2.00x109 N/m2 )

Ans: 1.65 x 108 N/m2

Explaining the inertia ball tensions.

• How is ΔL affected when you have two same springs in series or in parallel pulled by the same force?