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Gravitation question

2005 · Shift 0 · Q143
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Gravitation question

2005 · Shift 0 · Q143

JEE MainPhysicsGravitationMCQ+4 / −1
Average density of the earth
  1. A
    is a complex function of ggg
  2. B
    does not depend on ggg
  3. C
    is inversely proportional to ggg
  4. D
    is directly proportional to ggg
View written solutionFree

Correct answer: D

  1. Use the relation for gravitational acceleration at the surface of Earth

    g=GMR2g = \frac{GM}{R^2}g=R2GM​

    where:

    • GGG = gravitational constant
    • MMM = mass of Earth
    • RRR = radius of Earth
  2. Write mass in terms of average density

    Average density of Earth is

    ρ=M43πR3\rho = \frac{M}{\frac{4}{3}\pi R^3}ρ=34​πR3M​

    So,

    M=ρ⋅43πR3M = \rho \cdot \frac{4}{3}\pi R^3M=ρ⋅34​πR3

  3. Substitute into the expression for ggg

    g=G(ρ⋅43πR3)R2g = \frac{G\left(\rho \cdot \frac{4}{3}\pi R^3\right)}{R^2}g=R2G(ρ⋅34​πR3)​

    Simplifying,

    g=43πGρRg = \frac{4}{3}\pi G \rho Rg=34​πGρR

  4. Express density in terms of ggg

    ρ=3g4πGR\rho = \frac{3g}{4\pi G R}ρ=4πGR3g​

  5. Interpret the result

    Since GGG and RRR are constants for Earth,

    ρ∝g\rho \propto gρ∝g

    Therefore, the average density of the Earth is directly proportional to ggg.

  6. Check options

    • A: False
    • B: False
    • C: False
    • D: True

Hence, the correct option is D.

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