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Gravitation question

2020 · 8 Jan · Shift 1 · Q46
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Gravitation question

2020 · 8 Jan · Shift 1 · Q46

JEE MainPhysicsGravitationMCQ+4 / −1
Consider two solid spheres of radii R1 = 1m, R2 = 2m and masses M1 and M2, respectively. The gravitational field due to sphere (1) and (2) are shown. The value of M1M2{{{M_1}} \over {{M_2}}}M2​M1​​ is : JEE Main 2020 (Online) 8th January Morning Slot Physics - Gravitation Question 151 English
  1. A
    23{2 \over 3}32​
  2. B
    16{1 \over 6}61​
  3. C
    12{1 \over 2}21​
  4. D
    13{1 \over 3}31​
View written solutionFree

Correct answer: B

  1. Gravitational field of a uniform solid sphere

For a solid sphere of radius RRR and mass MMM:

  • Inside the sphere (r<R)(r<R)(r<R), g(r)=GMR3rg(r)=\frac{GM}{R^3}rg(r)=R3GM​r so the field increases linearly from the center to the surface.

  • At the surface (r=R)(r=R)(r=R), gsurface=GMR2g_{\text{surface}}=\frac{GM}{R^2}gsurface​=R2GM​

  • Outside the sphere (r>R)(r>R)(r>R), g(r)=GMr2g(r)=\frac{GM}{r^2}g(r)=r2GM​

Thus, in the graph of ggg vs. rrr, the maximum value occurs at the surface.


  1. Use the graph information

Since the statement says that the gravitational fields due to sphere (1) and sphere (2) are shown, the relevant comparison is at their maxima, i.e. at their surfaces.

From the graph, the maximum field of sphere (1) is equal to the maximum field of sphere (2).

So, GM1R12=GM2R22\frac{GM_1}{R_1^2}=\frac{GM_2}{R_2^2}R12​GM1​​=R22​GM2​​

Cancel GGG: M1R12=M2R22\frac{M_1}{R_1^2}=\frac{M_2}{R_2^2}R12​M1​​=R22​M2​​

Hence, M1M2=R12R22\frac{M_1}{M_2}=\frac{R_1^2}{R_2^2}M2​M1​​=R22​R12​​

Given: R1=1 m,R2=2 mR_1=1\text{ m},\qquad R_2=2\text{ m}R1​=1 m,R2​=2 m

Therefore, M1M2=1222=14\frac{M_1}{M_2}=\frac{1^2}{2^2}=\frac{1}{4}M2​M1​​=2212​=41​


  1. Compare with options

The value obtained is M1M2=14\boxed{\frac{M_1}{M_2}=\frac{1}{4}}M2​M1​​=41​​

But 14\frac{1}{4}41​ is not present in the options.

So the stored answer 16\frac{1}{6}61​ cannot be correct for the standard gravitational-field graph interpretation.


  1. Conclusion

Using the standard formula for gravitational field of solid spheres and equating the surface values indicated by the graph, we get: M1M2=14\boxed{\frac{M_1}{M_2}=\frac{1}{4}}M2​M1​​=41​​

Hence I disagree with the stored answer.

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