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Units and Measurements question

2021 · 26 Aug · Shift 1 · Q60
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Units and Measurements question

2021 · 26 Aug · Shift 1 · Q60

JEE MainPhysicsUnits and MeasurementsMCQ+4 / −1
If E, L, M and G denote the quantities as energy, angular momentum, mass and constant of gravitation respectively, then the dimensions of P in the formula P = EL2M −-− 5G −-− 2 are :
  1. A
    [M0 L1 T0]
  2. B
    [M −-− 1 L −-− 1 T2]
  3. C
    [M1 L1 T −-− 2]
  4. D
    [M0 L0 T0]
View written solutionFree

Correct answer: D

  1. Interpret the formula

    The given expression is P=EL2M−5G−2P = E L^2 M^{-5} G^{-2}P=EL2M−5G−2 where:

    • EEE = energy
    • LLL = angular momentum
    • MMM = mass
    • GGG = gravitational constant
  2. Write dimensions of each quantity

    • Energy: [E]=[ML2T−2][E] = [M L^2 T^{-2}][E]=[ML2T−2]

    • Angular momentum: [L]=[ML2T−1][L] = [M L^2 T^{-1}][L]=[ML2T−1]

    • Mass: [M]=[M][M] = [M][M]=[M]

    • Gravitational constant: From Newton's law, F=Gm1m2r2F = \frac{G m_1 m_2}{r^2}F=r2Gm1​m2​​ so [G]=[F][r2][M]2=(MLT−2)(L2)M2=[M−1L3T−2][G] = \frac{[F][r^2]}{[M]^2} = \frac{(M L T^{-2})(L^2)}{M^2} = [M^{-1} L^3 T^{-2}][G]=[M]2[F][r2]​=M2(MLT−2)(L2)​=[M−1L3T−2]

  3. Compute dimensions of each factor in the formula

    [P]=[E][L]2[M]−5[G]−2[P] = [E][L]^2[M]^{-5}[G]^{-2}[P]=[E][L]2[M]−5[G]−2

    Substitute: [P]=(ML2T−2)(ML2T−1)2(M)−5(M−1L3T−2)−2[P] = (M L^2 T^{-2})(M L^2 T^{-1})^2 (M)^{-5} (M^{-1}L^3T^{-2})^{-2}[P]=(ML2T−2)(ML2T−1)2(M)−5(M−1L3T−2)−2

  4. Simplify term by term

    First, (ML2T−1)2=M2L4T−2(M L^2 T^{-1})^2 = M^2 L^4 T^{-2}(ML2T−1)2=M2L4T−2

    and (M−1L3T−2)−2=M2L−6T4(M^{-1}L^3T^{-2})^{-2} = M^2 L^{-6} T^4(M−1L3T−2)−2=M2L−6T4

    Therefore, [P]=(ML2T−2)(M2L4T−2)(M−5)(M2L−6T4)[P] = (M L^2 T^{-2})(M^2 L^4 T^{-2})(M^{-5})(M^2 L^{-6} T^4)[P]=(ML2T−2)(M2L4T−2)(M−5)(M2L−6T4)

  5. Add powers of M,L,TM, L, TM,L,T

    • Power of MMM: 1+2−5+2=01 + 2 - 5 + 2 = 01+2−5+2=0

    • Power of LLL: 2+4+0−6=02 + 4 + 0 - 6 = 02+4+0−6=0

    • Power of TTT: −2−2+0+4=0-2 - 2 + 0 + 4 = 0−2−2+0+4=0

    Hence, [P]=[M0L0T0][P] = [M^0 L^0 T^0][P]=[M0L0T0]

  6. Match with options

    [M0L0T0][M^0 L^0 T^0][M0L0T0] corresponds to Option D.

  7. Comparison with stored correct answer

    Stored correct answer = D

    Our derived answer = D

    So, they agree.

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