Physics · Properties of bulk matter · chapter 8 of 14

Class 11 Physics Chapter 8: Mechanical Properties of Solids MCQs with answers

10 multiple-choice questions on Mechanical Properties of Solids, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.

Question 1
The dimensional formula of stress is
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Answer: (b) [ML⁻¹T⁻²]

Stress = F/A, so [MLT⁻²]/[L²] = [ML⁻¹T⁻²]. [MLT⁻²] is force itself; [M⁰L⁰T⁰] belongs to strain.

Question 2
Which of the following has neither a unit nor a dimensional formula?
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Answer: (a) volume strain

Volume strain ΔV/V is a ratio of two volumes, so it has no unit or dimensions. Young's modulus and stress are in N m⁻²; compressibility, 1/B, is in m² N⁻¹.

Question 3
Two wires X and Y have the same length and the same area of cross-section. Under the same load, X stretches by 0.4 mm and Y by 0.9 mm. Which statement is correct?
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Answer: (b) X is more elastic, and has the larger Young's modulus.

With F, L and A the same, Y = FL/(AΔL) is larger for the smaller ΔL, so X has the larger Young's modulus. In physics the material that stretches less for a given load is the more elastic one; calling Y more elastic is the everyday misnomer.

Question 4
In a tensile test on a rod of a new ceramic, the rod fractures almost as soon as the stress reaches the ultimate tensile strength. The material is
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Answer: (c) brittle

When the ultimate strength point D and the fracture point E are close, the material is brittle. A ductile material shows a long stretch between D and E.

Question 5
Which statement about the elastic tissue of the aorta is correct?
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Answer: (d) Its elastic region is very large, but it does not obey Hooke's law over most of it.

NCERT notes that for aortic tissue the elastic region is very large, Hooke's law fails over most of it, and there is no well defined plastic region; such materials are elastomers.

Question 6
Which elastic modulus can be defined for water in a closed vessel?
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Answer: (c) bulk modulus

Young's and shear moduli need a definite length and shape, which only solids have. Bulk modulus refers to change of volume under uniform pressure and applies to solids, liquids and gases.

Question 7
A new alloy has Young's modulus 120 GPa. Its shear modulus is expected to be about
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Answer: (d) 40 GPa

For most materials G ≈ Y/3 = 120/3 = 40 GPa. 360 GPa multiplies instead of dividing; shear modulus is generally less than Young's modulus.

Question 8
The compressibility of solids, liquids and gases, in increasing order, is
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Answer: (b) solid < liquid < gas

Bulk modulus is largest for solids and smallest for gases, and compressibility is 1/B, so solid < liquid < gas: atoms in solids are most tightly coupled, gas molecules least.

Question 9
On the stress–strain curve of a metal, the yield point is also known as the
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Answer: (a) elastic limit

The yield point B is also called the elastic limit; the stress there is the yield strength σ_y. The proportional limit A comes earlier, and D and E come later.

Question 10
A wire of cross-section 2.0 mm² hangs from a ceiling with a 100 N weight at its lower end. Neglecting the wire's own weight, the tensile stress at its midpoint is
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Answer: (a) 50 MPa

The tension at any cross-section is just F = 100 N, not 2F, so stress = 100/(2.0 × 10⁻⁶) = 5.0 × 10⁷ Pa = 50 MPa. 100 MPa adds the ceiling's pull to the weight.

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