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JEE Main 202229 JulyMorning Shift
Exam
JEE Main
Year
2022
Date
29 July
Shift
Morning Shift
Subject
Physics
Chapter
Units and Dimensions

JEE Main 2022 · 29 July · Morning Shift · Physics · Units and Dimensions

Given below are two statements: One is labelled as Assertion (A)...

Given below are two statements: One is labelled as Assertion (A) and other is labelled as Reason (R)

Assertion (A) : Time period of oscillation of a liquid drop depends on surface tension (S)\left(S\right), if density of the liquid is ρρ and radius of the drop is rr, then T=Kρr3S32T = K\sqrt{\dfrac{ρ{r}^{3}}{{S}^{\dfrac{3}{2}}}} is dimensionally correct, where KK is dimensionless.

Reason (R) : Using dimensional analysis we get R.H.S. having different dimension than that of time period. In the light of above statements, choose the correct answer from the options given below.

Options

  1. ABoth (A) and (R) are true and (R) is the correct explanation of (A)
  2. BBoth (A) and (R) are true but (R) is not the correct explanation of (A)
  3. C(A) is true but (R) is false
  4. D(A) is false but (R) is true

Correct answer

D. (A) is false but (R) is true

Step-by-step solution

Using dimensional analysis for the given equation,

T=Kρr3S32T = K\sqrt{\dfrac{ρ{r}^{3}}{{S}^{\dfrac{3}{2}}}}

⇒[K]=[T][S34][ρ12][r32]⇒[K]=[T][M34T−32][M12L−32][L32]⇒[K]=[M14T−12]⇒ \left[K\right] = \dfrac{\left[T\right]\left[{S}^{\dfrac{3}{4}}\right]}{\left[{ρ}^{\dfrac{1}{2}}\right]\left[{r}^{\dfrac{3}{2}}\right]} ⇒ \left[K\right] = \dfrac{\left[T\right]\left[{M}^{\dfrac{3}{4}}{T}^{- \dfrac{3}{2}}\right]}{\left[{M}^{\dfrac{1}{2}}{L}^{- \dfrac{3}{2}}\right]\left[{L}^{\dfrac{3}{2}}\right]} ⇒ \left[K\right] = \left[M\dfrac{1}{4}{T}^{- \dfrac{1}{2}}\right]

Therefore, KK is not dimensionless. Hence, Assertion is false.

For dimensional validity of the given equation, KK can not be dimensionless. If it is dimensionless then

Dimensions of L.H.S ≠\ne Dimensions of R.H.S

∴∴ Option (D) is correct.

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