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Dimensional Analysis and Similitude Questions and Answers

11.

A set of basic dimensions, mass, length, and time are represented by M, L, and T, respectively. What will be the dimensions of pressure in M-L-T system?

Exam Year(s)

GATE XE - 2024,

Answer: A

Pressure has dimensions \(ML^{-1}T^{-2}\).

Pressure is force per unit area. Force is mass times acceleration, \(MLT^{-2}\), and area is \(L^{2}\):

\[ [p] = \frac{MLT^{-2}}{L^{2}} = ML^{-1}T^{-2} \]

\(MLT^{-2}\) is force itself, \(MLT^{-1}\) is momentum, and \(ML^{-1}T^{-1}\) is dynamic viscosity (Pa s).

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12.

The basic dimensions, i.e., mass, length, and time are represented by M, L, and T, respectively. The correct dimension of dynamic viscosity is

Exam Year(s)

GATE XE - 2026,

Answer: C

The dimension of dynamic viscosity is \(ML^{-1}T^{-1}\).

From Newton's law of viscosity, \(\tau = \mu\,du/dy\):

\[ \begin{aligned} [\tau] &= \frac{MLT^{-2}}{L^{2}} = ML^{-1}T^{-2} \\ [du/dy] &= T^{-1} \\ [\mu] &= ML^{-1}T^{-1} \end{aligned} \]

This matches the SI unit Pa.s, which is kg.m-1.s-1. \(MLT^{-2}\) is the dimension of force, and \(L^{2}T^{-1}\) is kinematic viscosity, \(\nu = \mu/\rho\), which has no mass dimension. \(L^{-2}T^{2}\) corresponds to no standard fluid property.

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13.

Match List - I with List - II :

List - IList - II
(a) Mach number(i) Inertia force/Gravitational force
(b) Froude number(ii) Inertia force/Elastic force
(c) Nusselt number(iii) Momentum Diffusivity/Thermal Diffusivity
(d) Prandtl number(iv) Convective heat transfer rate/Conductive heat transfer rate

Choose the most appropriate answer from the options given below :

Exam Year(s)

RSMSSB JE - 2025, RSMSSB - 2025,

Answer: B

The correct matching is (a)-(ii), (b)-(i), (c)-(iv), (d)-(iii).

  • Mach number is the square root of inertia force over elastic (compressibility) force, \(V/c\) (a-ii).
  • Froude number is the square root of inertia force over gravity force, \(V/\sqrt{gL}\) (b-i).
  • Nusselt number is convective over conductive heat transfer, \(hL/k\) (c-iv).
  • Prandtl number is momentum diffusivity over thermal diffusivity, \(\nu/\alpha\) (d-iii).

Option C swaps Nusselt and Prandtl, and options A and D give the Mach number the Prandtl number's ratio, so only option B has all four right.

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