1.Units, Dimensions and Measurement
hard

A highly rigid cubical block $A$ of small mass $M$ and side $L$ is fixed rigidly onto another cubical block $B$ of the same dimensions and of low modulus of rigidity $\eta $ such that the lower face of $A$ completely covers the upper face of $B$. The lower face of $B$is rigidly held on a horizontal surface. A small force $F$ is applied perpendicular to one of the side faces of $A$. After the force is withdrawn block $A$ executes small oscillations. The time period of which is given by

A

$2\pi \sqrt {\frac{{M\eta }}{L}} $

B

$2\pi \sqrt {\frac{L}{{M\eta }}} $

C

$2\pi \sqrt {\frac{{ML}}{\eta }} $

D

$2\pi \sqrt {\frac{M}{{\eta L}}} $

(IIT-1992)

Solution

(d) By substituting the dimensions of mass $[M]$, length $[L] $ and coefficient of rigidity $\left[ {M{L^{ – 1}}{T^{ – 2}}} \right]$ we get $T = 2\pi \sqrt {\frac{M}{{\eta L}}} $ is the right formula for time period of oscillations

$(a, b, c)$ Reynolds number and coefficient of friction are dimensionless.

Latent heat and gravitational potential both have dimension $[{L^2}{T^{ – 2}}]$.

Curie and frequency of a light wave both have dimension $[{T^{ – 1}}]$.

But dimensions of Planck's constant is $[M{L^2}{T^{ – 1}}]$ and torque is $\left[ {M{L^2}{T^{ – 2}}} \right]$

Standard 11
Physics

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