Ch: Mechanical properties of solids

NCERT Physics Grade 11

Deforming force

it is the force which can change the shape and size of an object.

Restoring force

the force developed inside the body against deforming force.

Elasticity

the property of a body to return to its original shape and size after the removal of a deforming force.

Plasticity

when a body doesn’t regain its original shape and size even after the removal of the deforming force.

Stress

internal restoring force set up per unit area of cross-section of deforming body. stress = F/A where F is force applied and A is area of cross-section.

Types of stress

• tensile stress: change in length in direction of deforming force

• volume stress: uniform force from all sides; change in volume.

• tangential stress: force acts tangentially to the surface of the body; change in shape

Strain

ratio of change in dimension produced in the body to original dimension. strain = change in dimension/ original dimension .

Types of strain

• longitudinal strain: Δl/l

• volume strain: ΔV/V

• shear strain: Δx/l

Elastic limit

max stress within which the body regains its original shape after removing deforming force.

Hooke’s Law

for small deformations stress is directly proportional to strain. stress ∝ strain
stress = k strain where k is modulus of elasticity k = stress/strain.

3 modulus’ (Hooke’s law)

• Young’s modulus (Y): Fl/AΔl

• Bulk modulus (B): FV/AΔV

• Sheer modulus (G): Fl/AΔx

ductile vs. brittle material

ductile materials are materials with large plastic range while brittle materials have small plastic range.

Elastomers

materials which can be elastically stretched to large values of strain with little stress.

Elastic after effect

delay in regaining original state of a body on removal of deforming force.

Elastic fatigue

loss in the strength in the material caused due to repeated alternating strains to which the material is subjected.

Elastic energy

work done against the interatomic forces when a wire is put under longitudinal stress is known as elastic energy.

Poisson’s ratio

Ratio of lateral to longitudinal strain.

σ = Δdl/dΔl