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Gravitation question

2019 · 9 Jan · Shift 2 · Q64
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Gravitation question

2019 · 9 Jan · Shift 2 · Q64

JEE MainPhysicsGravitationMCQ+4 / −1
The energy required to take a satellite to a height 'h' above Earth surface (radius of Earth = 6.4 ×\times× 103 km) is E1 and kinetic energy required for the satellite to be in a circular orbit at this height is E2. The value of h for which E1 and E2 are equal, is
  1. A
    1.6 ×\times× 103 km
  2. B
    3.2 ×\times× 103 km
  3. C
    6.4 ×\times× 103 km
  4. D
    1.28 ×\times× 104 km
View written solutionFree

Correct answer: B

  1. Given

    • Radius of Earth: R=6.4×103 kmR = 6.4 \times 10^3\ \text{km}R=6.4×103 km
    • Satellite is taken to height hhh above Earth, so orbital radius is r=R+hr = R + hr=R+h
  2. Energy required to take the satellite from Earth surface to height hhh

    The increase in gravitational potential energy is E1=GMm(1R−1r)E_1 = GMm\left(\frac{1}{R} - \frac{1}{r}\right)E1​=GMm(R1​−r1​)

  3. Kinetic energy of satellite in circular orbit at radius rrr

    For circular orbit, mv2r=GMmr2⇒v2=GMr\frac{mv^2}{r} = \frac{GMm}{r^2} \Rightarrow v^2 = \frac{GM}{r}rmv2​=r2GMm​⇒v2=rGM​

    Hence kinetic energy is E2=12mv2=GMm2rE_2 = \frac{1}{2}mv^2 = \frac{GMm}{2r}E2​=21​mv2=2rGMm​

  4. Condition given: E1=E2E_1 = E_2E1​=E2​

    So, GMm(1R−1r)=GMm2rGMm\left(\frac{1}{R} - \frac{1}{r}\right) = \frac{GMm}{2r}GMm(R1​−r1​)=2rGMm​

    Cancel GMmGMmGMm: 1R−1r=12r\frac{1}{R} - \frac{1}{r} = \frac{1}{2r}R1​−r1​=2r1​

    Rearranging, 1R=1r+12r=32r\frac{1}{R} = \frac{1}{r} + \frac{1}{2r} = \frac{3}{2r}R1​=r1​+2r1​=2r3​

    Therefore, r=3R2r = \frac{3R}{2}r=23R​

  5. Find height hhh

    Since r=R+h=3R2r = R + h = \frac{3R}{2}r=R+h=23R​ we get h=3R2−R=R2h = \frac{3R}{2} - R = \frac{R}{2}h=23R​−R=2R​

    Now substitute R=6.4×103 kmR = 6.4 \times 10^3\ \text{km}R=6.4×103 km: h=6.4×1032=3.2×103 kmh = \frac{6.4 \times 10^3}{2} = 3.2 \times 10^3\ \text{km}h=26.4×103​=3.2×103 km

  6. Option check

    • A: 1.6×1031.6 \times 10^31.6×103 km ✗
    • B: 3.2×1033.2 \times 10^33.2×103 km ✓
    • C: 6.4×1036.4 \times 10^36.4×103 km ✗
    • D: 1.28×1041.28 \times 10^41.28×104 km ✗

Therefore, the correct answer is Option B.

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