MCQEasyJEE 2025Dimensions & Dimensional Analysis

JEE Physics 2025 Question with Solution

Match List-I with List-II.

Table showing List-I and List-II for matching: Young’s Modulus, Torque, Coefficient of Viscosity, and Gravitational Constant with dimensional formula entries labeled I, II, III, and IV.

Choose the correct answer from the options given below:

  • A

    (A)-(I), (B)-(III), (C)-(I), (D)-(II)

  • B

    (A)-(II), (B)-(IV), (C)-(II), (D)-(III)

  • C

    (A)-(III), (B)-(I), (C)-(IV), (D)-(II)

  • D

    (A)-(I), (B)-(II), (C)-(I), (D)-(IV)

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: Match the physical quantities in List-I with the dimensional formulas in List-II.

Find: The correct matching and hence the correct option.

Use dimensional analysis for each quantity.

For Young's Modulus:

Young’s Modulus=stressstrain\text{Young's Modulus} = \frac{\text{stress}}{\text{strain}}

Since strain is dimensionless,

[Young’s Modulus]=[stress]=[Force][Area]=MLT2L2=ML1T2[\text{Young's Modulus}] = [\text{stress}] = \frac{[\text{Force}]}{[\text{Area}]} = \frac{MLT^{-2}}{L^2} = ML^{-1}T^{-2}

So, (A) \to (I).

For Torque:

[τ]=[Force]×[distance]=(MLT2)(L)=ML2T2[\tau] = [\text{Force}] \times [\text{distance}] = (MLT^{-2})(L) = ML^2T^{-2}

So, (B) \to (II).

For Coefficient of Viscosity: Using

η=Force/Areavelocity gradient\eta = \frac{\text{Force/Area}}{\text{velocity gradient}}

we get

[η]=ML1T2T1=ML1T1[\eta] = \frac{ML^{-1}T^{-2}}{T^{-1}} = ML^{-1}T^{-1}

So, (C) \to (I).

For Gravitational Constant: From Newton's law of gravitation,

F=Gm1m2r2F = G\frac{m_1m_2}{r^2}

Hence,

G=Fr2m1m2G = \frac{Fr^2}{m_1m_2}

and therefore

[G]=(MLT2)L2M2=M1L3T2[G] = \frac{(MLT^{-2})L^2}{M^2} = M^{-1}L^3T^{-2}

So, (D) \to (IV).

Therefore, the final matching is (A)-(I), (B)-(II), (C)-(I), (D)-(IV). The correct option is D.

Step-by-Step Matching

Given: Four physical quantities are to be matched with their dimensional formulas.

Find: The correct correspondence.

  1. Young's Modulus
  • It is stress divided by strain.
  • Strain has no dimensions.
  • Stress is force per unit area.
[stress]=MLT2L2=ML1T2[\text{stress}] = \frac{MLT^{-2}}{L^2} = ML^{-1}T^{-2}

Hence, (A) \to (I).

  1. Torque
  • Torque is force multiplied by perpendicular distance.
[torque]=(MLT2)(L)=ML2T2[\text{torque}] = (MLT^{-2})(L) = ML^2T^{-2}

Hence, (B) \to (II).

  1. Coefficient of Viscosity
  • From the relation involving viscous force per unit area,
Force per unit area=ηdvdy\text{Force per unit area} = \eta \frac{dv}{dy}
  • Since
[dvdy]=T1\left[\frac{dv}{dy}\right] = T^{-1}

we get

[η]=ML1T2T1=ML1T1[\eta] = \frac{ML^{-1}T^{-2}}{T^{-1}} = ML^{-1}T^{-1}

Hence, (C) \to (I).

  1. Gravitational Constant
  • From
F=Gm1m2r2F = G\frac{m_1m_2}{r^2}
  • Rearranging,
G=Fr2m1m2G = \frac{Fr^2}{m_1m_2}
  • Therefore,
[G]=(MLT2)L2M2=M1L3T2[G] = \frac{(MLT^{-2})L^2}{M^2} = M^{-1}L^3T^{-2}

Hence, (D) \to (IV).

So the required match is (A)-(I), (B)-(II), (C)-(I), (D)-(IV), which corresponds to option D.

Common mistakes

  • Confusing stress with force while finding Young's modulus is incorrect because Young's modulus is stress/strain, not force directly. Always divide force by area first, then note that strain is dimensionless.

  • Using the dimension of work incorrectly for torque without checking the derivation can cause errors in matching. Although torque and work have the same dimensions, here you should compute it directly as force × distance to avoid mismatch.

  • Taking the velocity gradient dvdy\frac{dv}{dy} as dimensionless is wrong because it has dimension T1T^{-1}. For viscosity, first write force per unit area, then divide by the velocity gradient carefully.

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