NCERT Solution Class 11th Physics Chapter – 9 Mechanical Properties of Solids
| Textbook | NCERT |
| class | Class – 11th |
| Subject | Physics |
| Chapter | Chapter – 9 |
| Chapter Name | Mechanical Properties of Solids |
| Category | Class 11th Physics Notes |
| Medium | English |
| Source | last doubt |
NCERT Solution Class 11th Physics Chapter – 9 Mechanical Properties of Solids Notes Elastic limit, Compression Strain, Brittle discuss Mechanical Properties of Solids Notes.
NCERT Solution Class 11th Physics Chapter – 9 Mechanical Properties of Solids
Chapter – 9
Mechanical Properties of Solids
Notes
Young’s modulus is defined only for solids.
Bulk modulus is defined for all types of materials solids, liquids, and gases.
Hook’s law is obeyed only for small values of strain (say of the order of 0.01).
Reciprocal the bulk modulus is called compressibility.
Within elastic limits, force constant for a spring is given by
k = YA/l
Higher values of elasticity mean the greater force is required for producing a given change.
The deformation beyond the elastic limit is called plasticity.
The materials which don’t break well beyond the elastic limitary called ductile.
The materials which break as soon as stress exceeds the elastic limit are called brittle.
Rubber sustains elasticity even when stretched several times its length However it is not ductile. It breaks down as soon as the elastic limit is crossed.
Quartz is the best example of a perfectly elastic material.
Stress and pressure have the same units and dimensions, but the pressure is always normal to the surface while the stress may be parallel or perpendicular to the surface.
When a body is sheared two mutually perpendicular strains are produced which are called longitudinal strain and compressional strain. Both are of equal magnitude.
Thermal stress in a rod = Y ∝ Δθ. It is independent of the area of cross-section or length of wire.
Breaking force depends on the area of the cross-section of the wire. Breaking stress per unit area of cross-section is also called tensile strength of a wire.
Elastic after effect is a temporary absence of the elastic properties.
Temporary loss of elastic properties due to continuous use for a long time is called elastic fatigue.
Normal stress is also called tensile stress when the length of the body tends to increase.
Normal stress is also called compressional stress when the length of the body tends to decrease.
Tangential stress is also called shearing stress.
When the deforming force is inclined to the surface, both tangential stress, as well as normal stress, are produced.
Diamond and Carborundum are the nearest approaches to a rigid body. Elasticity is the property of non-rigid bodies.
A negative value of Poisson’s ratio means that if length increases then the radius decreases.
If a beam of rectangular cross-section is loaded its depression is inversely proportional to the cube of thickness of the beard.
If a beam of circular cross-section is loaded, its depression is -inversely proportional to the cube of the radius.
If we double the radius of a wire its breaking load becomes four times and the breaking stress remains unchanged.
S.I. unit of stress is Nm-2 or Pascal (Pa).
The strain has no unit.
Breaking stress is independent of the length of the wire.
When a beam is bent, both extensional as well as compressional strain is produced.
Y is infinity for a perfectly rigid body and zero for air.
K for a perfectly rigid body is infinity and its compressibility is zero.
The modulus of rigidity of water is zero.
Solids – They are defined as substances that have definite shape and volume and have close packing of molecules.
Compression Strain – The reduction in dimension to the original dimension is called compression strain.
Compression Stress – The force per unit area which reduces the dimension of the body is called compression stress. It is maximum stress which the body can withstand on or before breaking.
Brittle – The materials in which yield point and breaking points are very close are called brittle.
Compressibility – It is defined as the reciprocal bulk modulus of elasticity.
Elastic limit – It is defined as the maximum stress on the removal of which, the body regains its original configuration.
Modulus of elasticity – It is defined as the ratio of stress to strain.
Important Formulae –
Young’s Modulus is given by
Y = F/AΔL/L=FL/AΔL
Bulk Modulus is given by
K = p−/ΔV/V=pV−ΔV/=−FV/AΔV
when -ve sign shows that volume decreases when pressure is applied.
Compressibility = 1/K=ΔV/pV
Modulus of rigidity is given by
η = T/θ =T(x/L)=TL/x =FL/Ax where T = F/A = tangential stress.
Work done per unit volume of wire = 1/2 stress × strain.
Work done to stretch a wire = 1/2 × stretching force × extension
= 1/2YAl2/ L
l = extension.