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Gravitation question

2024 · 9 Apr · Shift 2 · Q65
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  5. /2024 · 9 Apr · Shift 2 · Q65

Gravitation question

2024 · 9 Apr · Shift 2 · Q65

JEE MainPhysicsGravitationMCQ+4 / −1
A satellite of 103 kg10^3 \mathrm{~kg}103 kg mass is revolving in circular orbit of radius 2R2 R2R. If 104R6 J\frac{10^4 R}{6} \mathrm{~J}6104R​ J energy is supplied to the satellite, it would revolve in a new circular orbit of radius (use g=10 m/s2,R=g=10 \mathrm{~m} / \mathrm{s}^2, R=g=10 m/s2,R= radius of earth)
  1. A
    4 R
  2. B
    6 R
  3. C
    2.5 R
  4. D
    3 R
View written solutionFree

Correct answer: B

  1. Total energy of a satellite in a circular orbit

For a satellite of mass mmm in a circular orbit of radius rrr, the total mechanical energy is

E=−GMm2rE = -\frac{GMm}{2r}E=−2rGMm​

Using GM=gR2GM = gR^2GM=gR2, this becomes

E=−gR2m2rE = -\frac{gR^2 m}{2r}E=−2rgR2m​


  1. Initial orbit

Given:

  • m=103 kgm = 10^3\,\text{kg}m=103kg
  • initial radius r1=2Rr_1 = 2Rr1​=2R
  • g=10 m/s2g = 10\,\text{m/s}^2g=10m/s2

So initial total energy is

E1=−gR2m2(2R)=−gRm4E_1 = -\frac{gR^2 m}{2(2R)} = -\frac{gRm}{4}E1​=−2(2R)gR2m​=−4gRm​

Substitute values:

E1=−10⋅R⋅1034=−2500R JE_1 = -\frac{10 \cdot R \cdot 10^3}{4} = -2500R\,\text{J}E1​=−410⋅R⋅103​=−2500RJ


  1. Energy supplied

Given supplied energy:

ΔE=104R6 J\Delta E = \frac{10^4 R}{6}\,\text{J}ΔE=6104R​J

This increases the total energy of the satellite.

Hence new total energy:

E2=E1+ΔEE_2 = E_1 + \Delta EE2​=E1​+ΔE

E2=−2500R+104R6E_2 = -2500R + \frac{10^4R}{6}E2​=−2500R+6104R​

Take LCM:

−2500R=−15000R6-2500R = -\frac{15000R}{6}−2500R=−615000R​

So,

E2=−15000R6+10000R6=−5000R6=−2500R3E_2 = -\frac{15000R}{6} + \frac{10000R}{6} = -\frac{5000R}{6} = -\frac{2500R}{3}E2​=−615000R​+610000R​=−65000R​=−32500R​


  1. New circular orbit radius

For new orbit radius r2r_2r2​,

E2=−gR2m2r2E_2 = -\frac{gR^2 m}{2r_2}E2​=−2r2​gR2m​

Substitute g=10g=10g=10, m=103m=10^3m=103:

E2=−10⋅R2⋅1032r2=−5000R2r2E_2 = -\frac{10\cdot R^2 \cdot 10^3}{2r_2} = -\frac{5000R^2}{r_2}E2​=−2r2​10⋅R2⋅103​=−r2​5000R2​

Set this equal to the value found:

−5000R2r2=−2500R3-\frac{5000R^2}{r_2} = -\frac{2500R}{3}−r2​5000R2​=−32500R​

Cancel minus sign:

5000R2r2=2500R3\frac{5000R^2}{r_2} = \frac{2500R}{3}r2​5000R2​=32500R​

Multiply both sides by r2r_2r2​:

5000R2=2500R3r25000R^2 = \frac{2500R}{3} r_25000R2=32500R​r2​

r2=5000R2⋅32500R=6Rr_2 = \frac{5000R^2 \cdot 3}{2500R} = 6Rr2​=2500R5000R2⋅3​=6R


  1. Option check

The new circular orbit radius is

6R\boxed{6R}6R​

So the correct option is B.

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