Mechanical Properties of Solids — Class 11 MCQs with Answers

Mechanical Properties of Solids — important questions & MCQs with answers (Class 11 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   SI unit of stress:

   AJouleBPascal (N/m²)CNewtonDDimensionless
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   Answer: Pascal (N/m²)

   Explanation: Stress is defined as force divided by area (σ = F/A), so its SI unit is the pascal (N/m²) — not the newton (N), which is just the force part, missing the area denominator.

2. Q2 of 6Easy

   Stress = ?

   AForce onlyBForce × AreaCForce / AreaDArea only
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   Answer: Force / Area

   Explanation: Stress is force acting per unit cross-sectional area, σ = F/A (N/m²) — not force alone, and not force multiplied by area, which would give completely different units.

3. Q3 of 6Medium

   Rod of length 2 m stretches by 1 mm. Strain:

   ACannot computeB2 × 10⁻³C0.5D5 × 10⁻⁴
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   Answer: 5 × 10⁻⁴

   Explanation: Convert 1 mm to 0.001 m, then strain = ΔL/L = 0.001/2 = 5×10⁻⁴. Inverting the ratio to L/ΔL instead of ΔL/L is the trap that produces a much larger, wrong value.

4. Q4 of 6Medium

   Wire (L = 2 m, A = 1 mm²) stretches 1 mm under 200 N force. Y:

   A4 × 10¹¹ PaB2 × 10¹¹ PaC8 × 10¹¹ PaD1 × 10¹¹ Pa
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   Answer: 4 × 10¹¹ Pa

   Explanation: Convert both mm values to metres: A = 1×10⁻⁶ m², ΔL = 1×10⁻³ m. Then Y = FL/(AΔL) = (200×2)/(10⁻⁶×10⁻³) = 4×10¹¹ Pa — mixing up A and ΔL in the denominator produces the other values.

5. Q5 of 6Hard

   Y = (F·L)/(A·ΔL). Increasing length doubles ΔL keeping F, A same. New Y:

   AHalvesBDoublesCSame (material property)DVariable
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   Answer: Same (material property)

   Explanation: Young's modulus depends only on the material, not the specimen's dimensions — doubling the length also doubles ΔL proportionally (F, A, Y unchanged), so Y itself stays exactly the same.

6. Q6 of 6Hard

   For incompressible fluid, bulk modulus is:

   AInfiniteBZeroCNegativeDVariable
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   Answer: Infinite

   Explanation: An incompressible fluid has essentially zero volumetric strain (ΔV/V → 0) under any applied pressure, so K = −ΔP/(ΔV/V) tends to infinity — a vanishing denominator drives this, not a vanishing numerator.

Mechanical Properties of Solids notes: key concepts (Class 11 Physics)

How real solids deform under load: stress and strain and their three types, Hooke's law and the elastic limit, the full stress-strain curve from proportional limit to fracture, the three elastic moduli, Poisson's ratio, the energy stored in a strained body, and what separates a brittle material from a ductile one.

Elasticity and plasticity

Elasticity is the tendency to recover the original shape once the load is removed; plasticity is permanent deformation that remains after unloading.

Stress

The internal restoring force per unit area developed inside a deformed body, measured in pascals. It has the same dimensions as pressure but is not the same quantity.

Types of stress

Tensile or compressive stress acts perpendicular to a surface, shearing stress acts along it, and hydraulic stress acts uniformly from all sides.

Strain

The fractional deformation — change in length, volume or shape divided by the original. It is a pure ratio and therefore has no units at all.

Hooke's law

Within the elastic limit, stress is directly proportional to strain, the constant of proportionality being the relevant modulus of elasticity.

Stress-strain curve

The load-response signature of a material, running from the linear proportional region through the elastic limit and yield point to the fracture point.

Proportional and elastic limits

The proportional limit is where the graph stops being a straight line; the elastic limit, slightly beyond it, is the last point from which the body still fully recovers.

Yield point and plastic region

Past the yield point the material deforms substantially with little extra load, and the deformation is permanent once unloaded.

Ultimate tensile strength

The maximum stress a material can withstand, the peak of the stress-strain curve. Beyond it the specimen necks down and then fractures.

Young's modulus

The ratio of tensile stress to longitudinal strain, measuring resistance to being stretched. Steel has a far higher value than rubber, so steel is the more elastic of the two.

Bulk modulus

The ratio of hydraulic stress to volume strain, measuring resistance to uniform compression. It is defined with a minus sign since a pressure rise reduces volume.

Compressibility

The reciprocal of the bulk modulus — how readily a substance is squeezed. Gases are highly compressible, liquids barely, and an ideal incompressible fluid has infinite K.

Shear modulus

The ratio of shearing stress to shearing strain, measuring resistance to a change in shape at constant volume. It is defined only for solids, since fluids cannot sustain shear.

Poisson's ratio

The ratio of lateral to longitudinal strain when a wire is stretched. It is dimensionless, typically about 0.2 to 0.4 for metals, and reaches 0.5 for an incompressible material.

Elastic strain energy

The work done in deforming a body, stored as potential energy. Per unit volume it equals half the product of stress and strain.

Brittle versus ductile

A brittle material such as glass fractures soon after the elastic limit with almost no plastic region; a ductile material such as copper stretches substantially first.