Class JEE Physics ALL Q #1153
COMPETENCY BASED
APPLY
4 Marks 2026 JEE Main 2026 (Online) 21st January Morning Shift MCQ SINGLE
Find dimensions of $\frac{A}{B}$ if $(P+\frac{A^{2}}{B})+\frac{1}{2}\rho V^{2}=$ constant, where $P\rightarrow$ pressure, $\rho\rightarrow$ density, $V$ speed.
(A) $ML^{1}T^{-4}$
(B) $ML^{-1}T^{-4}$
(C) $ML^{2}T^{-4}$
(D) $ML^{-1}T^{-2}$

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Step-by-Step Solution

According to the principle of dimensional homogeneity, each term in the given equation must have the same dimensions.

Given equation: $(P+\frac{A^{2}}{B})+\frac{1}{2}\rho V^{2}=$ constant

Therefore, the dimensions of $P$, $\frac{A^{2}}{B}$, and $\frac{1}{2}\rho V^{2}$ must be the same.

1. Dimensions of Pressure ($P$):

$[P] = ML^{-1}T^{-2}$

2. Dimensions of Density ($\rho$) and Velocity ($V$):

$[\rho] = ML^{-3}$

$[V] = LT^{-1}$

Therefore, the dimensions of $\frac{1}{2}\rho V^{2}$ are:

$[\frac{1}{2}\rho V^{2}] = [ML^{-3}][LT^{-1}]^{2} = ML^{-3}L^{2}T^{-2} = ML^{-1}T^{-2}$

3. Since $[P] = [\frac{A^{2}}{B}]$, we have:

$[\frac{A^{2}}{B}] = ML^{-1}T^{-2}$

We need to find the dimensions of $\frac{A}{B}$. Let's denote it as $[X] = [\frac{A}{B}]$

Then, $[\frac{A^{2}}{B}] = [A] \cdot [\frac{A}{B}] = [A] \cdot [X] = ML^{-1}T^{-2}$

4. Also, since $[P] = [\frac{1}{2}\rho V^{2}]$, we can equate the dimensions:

$P = \frac{A^2}{B}$

$\frac{A^2}{B} = \rho V^2$

$\frac{A^2}{B} = ML^{-1}T^{-2}$

Now, let's consider $\frac{A}{B} = X$. Then $A = BX$. Substituting this into the previous equation:

$\frac{(BX)^2}{B} = ML^{-1}T^{-2}$

$B X^2 = ML^{-1}T^{-2}$

We also know that $\frac{A^2}{B}$ has the same dimensions as pressure, so $\frac{A^2}{B} = ML^{-1}T^{-2}$.

Let's find the dimensions of A first. Since $\frac{A^2}{B}$ has the same dimensions as pressure and $\frac{1}{2}\rho V^2$, we have:

$\frac{A^2}{B} \sim ML^{-1}T^{-2}$

Now, consider the term $\frac{1}{2}\rho V^2$. Its dimensions are $ML^{-1}T^{-2}$. Therefore, $\frac{A^2}{B}$ must also have these dimensions.

Let's assume $\frac{A}{B} = X$. Then $A = BX$. So, $\frac{(BX)^2}{B} = BX^2 = ML^{-1}T^{-2}$.

We need to find X. Let's try another approach.

Since $P$ and $\frac{1}{2}\rho V^2$ have the same dimensions, we can write:

$P = \frac{A^2}{B}$

$ML^{-1}T^{-2} = \frac{A^2}{B}$

We want to find $\frac{A}{B}$. Let's rewrite the equation as:

$A^2 = B(ML^{-1}T^{-2})$

$A = \sqrt{B(ML^{-1}T^{-2})}$

Now, $\frac{A}{B} = \frac{\sqrt{B(ML^{-1}T^{-2})}}{B} = \sqrt{\frac{ML^{-1}T^{-2}}{B}}$

This approach doesn't seem to simplify things. Let's go back to $BX^2 = ML^{-1}T^{-2}$.

Since $\frac{A^2}{B}$ has the same dimensions as $\rho V^2$, we have $\frac{A^2}{B} = ML^{-1}T^{-2}$.

Let $X = \frac{A}{B}$. Then $A = BX$. Substituting this into the equation above:

$\frac{(BX)^2}{B} = ML^{-1}T^{-2}$

$BX^2 = ML^{-1}T^{-2}$

We need to find $X = \frac{A}{B}$. Let's try to express $A$ in terms of known quantities.

From $\frac{A^2}{B} = ML^{-1}T^{-2}$, we have $A^2 = B(ML^{-1}T^{-2})$.

Now, let's consider the dimensions of $B$. We know that $\frac{A^2}{B}$ has the dimensions of pressure, which is $ML^{-1}T^{-2}$.

Let's assume $A$ has dimensions $M^aL^bT^c$ and $B$ has dimensions $M^xL^yT^z$.

Then $\frac{(M^aL^bT^c)^2}{M^xL^yT^z} = ML^{-1}T^{-2}$.

$\frac{M^{2a}L^{2b}T^{2c}}{M^xL^yT^z} = ML^{-1}T^{-2}$.

$M^{2a-x}L^{2b-y}T^{2c-z} = ML^{-1}T^{-2}$.

So, $2a-x = 1$, $2b-y = -1$, and $2c-z = -2$.

We want to find $\frac{A}{B}$, which has dimensions $\frac{M^aL^bT^c}{M^xL^yT^z} = M^{a-x}L^{b-y}T^{c-z}$.

Let $a-x = p$, $b-y = q$, and $c-z = r$. We want to find $M^pL^qT^r$.

We have $2a-x = 1$, so $a = \frac{1+x}{2}$. Then $p = a-x = \frac{1+x}{2} - x = \frac{1-x}{2}$.

We have $2b-y = -1$, so $b = \frac{-1+y}{2}$. Then $q = b-y = \frac{-1+y}{2} - y = \frac{-1-y}{2}$.

We have $2c-z = -2$, so $c = \frac{-2+z}{2}$. Then $r = c-z = \frac{-2+z}{2} - z = \frac{-2-z}{2}$.

So, we want to find $M^{\frac{1-x}{2}}L^{\frac{-1-y}{2}}T^{\frac{-2-z}{2}}$.

Let's consider the given options. We have $ML^{-1}T^{-4}$. This means $x = -1, y = 1, z = 8$.

Then $\frac{1-x}{2} = \frac{1-(-1)}{2} = 1$. $\frac{-1-y}{2} = \frac{-1-1}{2} = -1$. $\frac{-2-z}{2} = \frac{-2-8}{2} = -5$.

So, we have $ML^{-1}T^{-5}$, which is not in the options.

Let's try to find the dimensions of A and B separately. We know that $\frac{A^2}{B}$ has the dimensions of pressure, $ML^{-1}T^{-2}$.

Let's assume $B$ has the dimensions of area, $L^2$. Then $A^2$ has the dimensions of $ML^{-1}T^{-2}L^2 = MLT^{-2}$. So, $A$ has the dimensions of $M^{1/2}L^{1/2}T^{-1}$.

Then $\frac{A}{B}$ has the dimensions of $\frac{M^{1/2}L^{1/2}T^{-1}}{L^2} = M^{1/2}L^{-3/2}T^{-1}$. This is not in the options.

Since $P$ and $\frac{1}{2}\rho V^2$ have the same dimensions, we have $ML^{-1}T^{-2}$. Thus, $\frac{A^2}{B}$ must also have these dimensions.

Let's assume $A$ has dimensions $M^aL^bT^c$ and $B$ has dimensions $M^xL^yT^z$. Then $\frac{A}{B}$ has dimensions $M^{a-x}L^{b-y}T^{c-z}$.

We know that $\frac{A^2}{B}$ has dimensions $ML^{-1}T^{-2}$. So, $\frac{(M^aL^bT^c)^2}{M^xL^yT^z} = ML^{-1}T^{-2}$.

This means $2a-x = 1$, $2b-y = -1$, and $2c-z = -2$.

We want to find $\frac{A}{B}$, which has dimensions $M^{a-x}L^{b-y}T^{c-z}$.

Let's try option (A): $MLT^{-4}$. Then $a-x = 1$, $b-y = 1$, and $c-z = -4$.

We have $2a-x = 1$, so $a = \frac{1+x}{2}$. Then $a-x = \frac{1+x}{2} - x = \frac{1-x}{2} = 1$. So, $1-x = 2$, which means $x = -1$.

We have $2b-y = -1$, so $b = \frac{-1+y}{2}$. Then $b-y = \frac{-1+y}{2} - y = \frac{-1-y}{2} = 1$. So, $-1-y = 2$, which means $y = -3$.

We have $2c-z = -2$, so $c = \frac{-2+z}{2}$. Then $c-z = \frac{-2+z}{2} - z = \frac{-2-z}{2} = -4$. So, $-2-z = -8$, which means $z = 6$.

So, $B$ has dimensions $M^{-1}L^{-3}T^6$. Then $A$ has dimensions $M^0L^{-2}T^2$.

Then $\frac{A^2}{B}$ has dimensions $\frac{(M^0L^{-2}T^2)^2}{M^{-1}L^{-3}T^6} = \frac{L^{-4}T^4}{M^{-1}L^{-3}T^6} = ML^{-1}T^{-2}$. This is correct.

So, $\frac{A}{B}$ has dimensions $\frac{M^0L^{-2}T^2}{M^{-1}L^{-3}T^6} = MLT^{-4}$.

Correct Answer: $ML^{1}T^{-4}$

AI Suggestion: Option A

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because it requires the student to apply the principle of dimensional homogeneity and the knowledge of dimensions of pressure, density, and velocity to find the dimensions of A/B.
Knowledge Dimension: CONCEPTUAL
Justification: The question tests the understanding of the concept of dimensional analysis and its application in determining the dimensions of physical quantities within a given equation. It requires understanding the underlying principles rather than just recalling facts.
Syllabus Audit: In the context of JEE, this is classified as COMPETENCY. It requires application of dimensional analysis to solve a problem, which is a competency-based skill.