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Gravitation question

2020 · 3 Sep · Shift 1 · Q57
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  5. /2020 · 3 Sep · Shift 1 · Q57

Gravitation question

2020 · 3 Sep · Shift 1 · Q57

JEE MainPhysicsGravitationMCQ+4 / −1
A satellite is moving in a low nearly circular orbit around the earth. Its radius is roughly equal to that of the earth’s radius Re . By firing rockets attached to it, its speed is instantaneously increased in the direction of its motion so that it become 32\sqrt {{3 \over 2}}23​​ times larger. Due to this the farthest distance from the centre of the earth that the satellite reaches is R. Value of R is :
  1. A
    2Re
  2. B
    3Re
  3. C
    4Re
  4. D
    2.5Re
View written solutionFree

Correct answer: B

  1. Initial circular orbit

The satellite is in a low circular orbit of radius approximately equal to Earth's radius: r0=Rer_0 = R_er0​=Re​

For a circular orbit, speed is vc=GMRev_c = \sqrt{\frac{GM}{R_e}}vc​=Re​GM​​ where MMM is the mass of Earth.

  1. Speed after firing rockets

Its speed is increased instantaneously in the same direction to v=32 vcv = \sqrt{\frac{3}{2}}\,v_cv=23​​vc​ So, v2=32vc2=32⋅GMRev^2 = \frac{3}{2}v_c^2 = \frac{3}{2}\cdot \frac{GM}{R_e}v2=23​vc2​=23​⋅Re​GM​

  1. Nature of the new orbit

The velocity is increased tangentially, so the point where the impulse is given becomes the perigee (nearest point), since speed there is maximum.

Thus, rp=Rer_p = R_erp​=Re​ Let the farthest distance be R=raR = r_aR=ra​.

  1. Use conservation of mechanical energy

Total specific energy (energy per unit mass) just after the impulse is ε=v22−GMRe\varepsilon = \frac{v^2}{2} - \frac{GM}{R_e}ε=2v2​−Re​GM​ Substitute v2v^2v2: ε=12(32GMRe)−GMRe\varepsilon = \frac{1}{2}\left(\frac{3}{2}\frac{GM}{R_e}\right) - \frac{GM}{R_e}ε=21​(23​Re​GM​)−Re​GM​ ε=34GMRe−GMRe\varepsilon = \frac{3}{4}\frac{GM}{R_e} - \frac{GM}{R_e}ε=43​Re​GM​−Re​GM​ ε=−14GMRe\varepsilon = -\frac{1}{4}\frac{GM}{R_e}ε=−41​Re​GM​

For an elliptical orbit, ε=−GM2a\varepsilon = -\frac{GM}{2a}ε=−2aGM​ where aaa is the semi-major axis.

So, −GM2a=−14GMRe-\frac{GM}{2a} = -\frac{1}{4}\frac{GM}{R_e}−2aGM​=−41​Re​GM​ Cancel −GM-GM−GM: 12a=14Re\frac{1}{2a} = \frac{1}{4R_e}2a1​=4Re​1​ 2a=4Re2a = 4R_e2a=4Re​ a=2Rea = 2R_ea=2Re​

  1. Relate semi-major axis to nearest and farthest distances

For an ellipse, a=rp+ra2a = \frac{r_p + r_a}{2}a=2rp​+ra​​ Thus, 2Re=Re+R22R_e = \frac{R_e + R}{2}2Re​=2Re​+R​ 4Re=Re+R4R_e = R_e + R4Re​=Re​+R R=3ReR = 3R_eR=3Re​

  1. Check options
  • A: 2Re2R_e2Re​ ❌
  • B: 3Re3R_e3Re​ ✅
  • C: 4Re4R_e4Re​ ❌
  • D: 2.5Re2.5R_e2.5Re​ ❌

Therefore, the farthest distance from the center of Earth is 3Re\boxed{3R_e}3Re​​

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