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JEE Advanced 2020 Paper 2 · Physics · Gravitation · hard

A satellite moves in a circular orbit of radius r about a planet of mass M (speed v0, period T0). By a short rocket burn its speed is suddenly raised…

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A satellite moves in a circular orbit of radius rr about a planet of mass MM (speed v0v_0, period T0T_0). By a short rocket burn its speed is suddenly raised to 1.2 v01.2\,v_0 along the direction of motion. Choose the correct statement(s).
  1. A. The new orbit is an ellipse with farthest distance 18r/718r/7 from the planet's centre.
  2. B. The speed at the farthest point is 715v0\tfrac{7}{15}v_0.
  3. C. The new orbital period is about 2.39 T02.39\,T_0.
  4. D. The total mechanical energy of the satellite in the new orbit is positive.
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Answer: A, B, C

With v02=GM/rv_0^2=GM/r, the new specific energy is ε=12(1.44)v02−v02=−0.28 GM/r\varepsilon=\tfrac12(1.44)v_0^2-v_0^2=-0.28\,GM/r. For an ellipse ε=−GM/2a\varepsilon=-GM/2a, so a=r/0.56=25r/14a=r/0.56=25r/14. The burn point is the nearest point (velocity is perpendicular to the radius), so the farthest distance is ra=2a−r=18r/7r_a=2a-r=18r/7: (A) is true. Angular momentum conservation gives vara=1.2v0rv_ar_a=1.2v_0r, so va=1.2×7/18 v0=7v0/15v_a=1.2\times7/18\,v_0=7v_0/15: (B) is true. Kepler's third law gives T/T0=(a/r)3/2=(25/14)1.5≈2.39T/T_0=(a/r)^{3/2}=(25/14)^{1.5}\approx2.39: (C) is true. The energy is negative (−0.28 GM/r-0.28\,GM/r, since 1.2<21.2<\sqrt2 the escape speed ratio), so the orbit is bound: (D) is false. Kepler's second law is consistent with this: the satellite moves slowest at the farthest point, where the speed is only 7/15≈0.477/15\approx0.47 of the original circular speed, and fastest at the burn point where it began.
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