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Gravitation question

2024 · 4 Apr · Shift 2 · Q76
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  5. /2024 · 4 Apr · Shift 2 · Q76

Gravitation question

2024 · 4 Apr · Shift 2 · Q76

JEE MainPhysicsGravitationMCQ+4 / −1
Correct formula for height of a satellite from earths surface is :
  1. A
    (T2R2g4π2)1/3−R\left(\frac{T^2 R^2 g}{4 \pi^2}\right)^{1 / 3}-R(4π2T2R2g​)1/3−R
  2. B
    (T2R2g4π)1/2−R\left(\frac{T^2 R^2 g}{4 \pi}\right)^{1 / 2}-R(4πT2R2g​)1/2−R
  3. C
    (T2R2g4π2)−1/3+R\left(\frac{T^2 R^2 g}{4 \pi^2}\right)^{-1 / 3}+R(4π2T2R2g​)−1/3+R
  4. D
    (T2R24π2g)1/3−R\left(\frac{T^2 R^2}{4 \pi^2 g}\right)^{1 / 3}-R(4π2gT2R2​)1/3−R
View written solutionFree

Correct answer: A

  1. Let the satellite orbit at a distance rrr from the center of the Earth.

    If hhh is the height from Earth’s surface, then r=R+hr=R+hr=R+h where RRR is Earth’s radius.

  2. For a satellite in circular orbit, gravitational force provides the centripetal force: GMmr2=mω2r\frac{GMm}{r^2}=m\omega^2 rr2GMm​=mω2r

    Using ω=2πT\omega=\frac{2\pi}{T}ω=T2π​, GMr2=(2πT)2r\frac{GM}{r^2}=\left(\frac{2\pi}{T}\right)^2 rr2GM​=(T2π​)2r

  3. Rearranging, GM=4π2r3T2GM=\frac{4\pi^2 r^3}{T^2}GM=T24π2r3​ so r3=GMT24π2r^3=\frac{GMT^2}{4\pi^2}r3=4π2GMT2​

  4. Now use the relation at Earth’s surface: g=GMR2g=\frac{GM}{R^2}g=R2GM​ hence GM=gR2GM=gR^2GM=gR2

    Substitute into the expression for r3r^3r3: r3=gR2T24π2r^3=\frac{gR^2T^2}{4\pi^2}r3=4π2gR2T2​

  5. Therefore, r=(T2R2g4π2)1/3r=\left(\frac{T^2R^2g}{4\pi^2}\right)^{1/3}r=(4π2T2R2g​)1/3

  6. Since height above Earth’s surface is h=r−Rh=r-Rh=r−R, h=(T2R2g4π2)1/3−Rh=\left(\frac{T^2R^2g}{4\pi^2}\right)^{1/3}-Rh=(4π2T2R2g​)1/3−R

  7. Compare with the options:

    • A: (T2R2g4π2)1/3−R\left(\frac{T^2 R^2 g}{4 \pi^2}\right)^{1 / 3}-R(4π2T2R2g​)1/3−R ✅
    • B: wrong power and wrong denominator
    • C: wrong sign of power and wrong final operation
    • D: ggg is in denominator, which is incorrect

Therefore, the correct option is A.

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