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Consider two physical quantities A and B related to each other as E = fracB - x^2At where E, x and t have dimensions of energy, length and time respectively. The dimension of AB is

Solution & Explanation

### Related Formula By the Principle of Homogeneity, terms added or subtracted must have the same dimensions: [B] = [x^2] ### Core Logic Known dimensional formulas: Length x to [L] Energy E to [ML^2T^-2] Time t to [T] ### Step 1: Dimension of B Since x^2 is subtracted from B: [B] = [x^2] = [L^2] ### Step 2: Dimension of A From the equation E = fracB - x^2At: [A] = frac[B - x^2][E][t] [A] = frac[L^2][ML^2T^-2][T] = frac[L^2][ML^2T^-1] [A] = [M^-1T^1] ### Step 3: Dimension of AB [AB] = [A] times [B] [AB] = [M^-1T^1] times [L^2] [AB] = [L^2 M^-1 T^1] ### Pattern Recognition Identify sums/differences first to instantly isolate B. Once [B] is fixed, the entire numerator is just L^2. Swap out variables to isolate [A]. Combining is just standard exponent addition. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Units and Measurements

Reference Study Guides

More Units and Measurements Previous-Year Questions — Page 6

Q33 jee_main_2024_29_january_evening Error Analysis
A physical quantity Q is found to depend on quantities a, b, c by the relation Q = fraca^4 b^3c^2. The percentage error in a, b and c are 3\%, 4\% and 5\% respectively. Then, the percentage error in Q is:
  • A. 66\%
  • B. 43\%
  • C. 34\%
  • D. 14\%

Solution

### Related Formula For a quantity Q = fraca^x b^yc^z, the maximum relative error is: fracDelta QQ = x fracDelta aa + y fracDelta bb + z fracDelta cc Multiplying by 100 gives the percentage error equation: \%text error in Q = x(\%text error in a) + y(\%text error in b) + z(\%text error in c) ### Core Logic Given formula: Q = fraca^4 b^3c^2 Applying the error propagation rule: fracDelta QQ times 100 = 4 left(fracDelta aa times 100right) + 3 left(fracDelta bb times 100right) + 2 left(fracDelta cc times 100right) ### Step 1: Substitute the Percentage Errors Substitute the given values: * \%text error in a = 3\% * \%text error in b = 4\% * \%text error in c = 5\% \%text error in Q = 4(3\%) + 3(4\%) + 2(5\%) \%text error in Q = 12\% + 12\% + 10\% = 34\% ### Pattern Recognition Error percentages always add up, weighted by their exponents in the mathematical expression, regardless of whether the variable is in the numerator or denominator. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Units and Measurements
Q38 jee_main_2024_27_jan_morning Measuring Instruments
Identify the physical quantity that cannot be measured using a spherometer:
  • A. textRadius of curvature of concave surface
  • B. textSpecific rotation of liquids
  • C. textThickness of thin plates
  • D. textRadius of curvature of convex surface

Solution

### Core Logic A spherometer is a mechanical instrument designed to measure small vertical displacements to calculate the thickness of thin plates or the radius of curvature of spherical (convex/concave) surfaces. Specific rotation of liquids is an optical property measured via a polarimeter, completely outside the scope of a spherometer. ### Pattern Recognition Spherometers operate purely on linear micrometer screw scale geometry, hence restricted strictly to spatial dimensions. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Units and Measurements
Q47 jee_main_2024_27_jan_morning Dimensional Analysis
Given below are two statements: Statement (I): Planck's constant and angular momentum have same dimensions. Statement (II): Linear momentum and moment of force have same dimensions. In the light of the above statements, choose the correct answer from the options given below:
  • A. textStatement I is true but Statement II is false
  • B. textBoth Statement I and Statement II are false
  • C. textBoth Statement I and Statement II are true
  • D. textStatement I is false but Statement II is true

Solution

### Core Logic Evaluate dimensions step-by-step: 1. **Planck's constant (h)**: E = h u implies [h] = frac[E][ u] = fractextML^2textT^-2textT^-1 = textML^2textT^-1 2. **Angular momentum (L)**: L = mvr implies [L] = textM cdot textLT^-1 cdot textL = textML^2textT^-1 Since [h] = [L], Statement I is true. 3. **Linear momentum (P)**: P = mv implies [P] = textMLT^-1 4. **Moment of force (Torque tau)**: tau = F r implies [tau] = textMLT^-2 cdot textL = textML^2textT^-2 Since [P] neq [tau], Statement II is false. ### Pattern Recognition Planck's constant can always be paired with angular momentum units (Joule-seconds), while moment of force matches work/energy footprints, not translational momentum. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Units and Measurements
Q38 jee_main_2024_29_jan_morning Error Analysis
The resistance R = fracVI where V = (200 pm 5) mathrm~V and I = (20 pm 0.2) mathrm~A, the percentage error in the measurement of R is:
  • A. 3.5%
  • B. 7%
  • C. 3%
  • D. 5.5%

Solution

### Related Formula By propagation of maximum relative error in division: R = fracVI implies fracDelta RR = fracDelta VV + fracDelta II Percentage error in R is given by: \% text error in R = left( fracDelta VV + fracDelta II right) times 100 ### Core Logic Given values: V = 200 mathrm~V, quad Delta V = 5 mathrm~V I = 20 mathrm~A, quad Delta I = 0.2 mathrm~A ### Step 1: Evaluate Relative Error fracDelta RR = frac5200 + frac0.220 fracDelta RR = frac5200 + frac2200 fracDelta RR = frac7200 ### Step 2: Calculate Percentage Error \% text error in R = fracDelta RR times 100 = frac7200 times 100 = 3.5\% Thus, the percentage error is 3.5\%. ### Pattern Recognition Whenever independent physical quantities are multiplied or divided, their fractional/relative errors always add up. Make sure to keep the base denominator values aligned to make mental calculations quick. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Units and Measurements
Q31 jee_main_2024_30_january_evening Vernier Callipers
If 50 Vernier divisions are equal to 49 main scale divisions of a travelling microscope and one smallest reading of main scale is 0.5 mathrm~mm, the Vernier constant of travelling microscope is:
  • A. 0.1 mathrm~mm
  • B. 0.1 mathrm~cm
  • C. 0.01 mathrm~cm
  • D. 0.01 mathrm~mm

Solution

### Related Formula textVernier Constant (Least Count) = 1 mathrm~MSD - 1 mathrm~VSD ### Core Logic Given that 50 Vernier Scale Divisions (VSD) equal 49 Main Scale Divisions (MSD). 50 mathrm~VSD = 49 mathrm~MSD 1 mathrm~VSD = frac4950 mathrm~MSD Also, the smallest reading of the main scale (1 mathrm~MSD) is 0.5 mathrm~mm. ### Step 1: Calculate Vernier Constant textVernier Constant = 1 mathrm~MSD - 1 mathrm~VSD = 1 mathrm~MSD - frac4950 mathrm~MSD = frac150 mathrm~MSD Substitute the value of 1 mathrm~MSD: = frac150 times 0.5 mathrm~mm = frac0.550 mathrm~mm = frac1100 mathrm~mm = 0.01 mathrm~mm ### Pattern Recognition In Vernier calipers problems where N text VSD = (N-1) text MSD, the Least Count is always exactly frac1N text MSD. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Units and Measurements

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