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Saturday, 31 October

JEE Main 2021 · PhysicsMultiple choiceDiagram basedMediumApplication

JEE Main 25 February 2021, Shift 1, Physics Q20

Question 20 of 90 in this shift, Physics question 20 of 30, Section A.

A solid sphere of radius R gravitationally attracts a particle placed at 3R from its centre with a force F1F_1. Now a spherical cavity of radius (R2)\left(\frac{R}{2}\right) is made in the sphere (as shown in figure) and the force becomes F2F_2. The value of F1:F2F_1 : F_2 is :

The figure, in words

A large hatched (shaded) circle labelled M with its centre marked O; inside it, touching its right edge, a smaller unshaded circle (the cavity) with its centre marked B. A horizontal line runs from O through B to a point A on the right, with a mass labelled m at A; a double-headed arrow labelled 2R spans from the right edge of the cavity to A.
  1. (1)41 : 50
  2. (2)50 : 41Official answer
  3. (3)36 : 25
  4. (4)25 : 36

Official answer

Option 2

NTA final key.

Chapter
Gravitation
Topic
Gravitational Potential
Idea tested
Potential Inside a Spherical Cavity

Same topic in other shifts

All Gravitational Potential questions
  1. 2 Apr 2026, Shift 2 · Q31If a body of mass 1 kg falls on the earth from infinity, it attains velocity (v) and kinetic energy (k) on reaching the surface of earth.…MediumSingle correct
  2. 5 Apr 2026, Shift 2 · Q32A body of mass mm is taken from the surface of earth to a height equal to twice the radius of earth (ReR_e). The increase in potential…EasySingle correct
  3. 4 Apr 2025, Shift 2 · Q29An object is kept at rest at a distance of 3R above the earth's surface where R is earth's radius. The minimum speed with which it must be…MediumSingle correct
  4. 6 Apr 2024, Shift 1 · Q33To project a body of mass m from earth's surface to infinity, the required kinetic energy is (assume, the radius of earth is RER_E, g =…EasySingle correct
  5. 30 Jan 2024, Shift 1 · Q36The gravitational potential at a point above the surface of earth is −5.12×107 J/kg-5.12\times10^{7}\ \mathrm{J/kg} and the acceleration due to…MediumSingle correct
  6. 11 Apr 2023, Shift 2 · Q37If V is the gravitational potential due to sphere of uniform density on it's surface, then it's value at the center of sphere will be:-MediumSingle correct

Question text from the official JEE Main paper published by NTA; answer from the final answer key. Chapter, topic and idea tags, difficulty and skill are Lumi’s. This question needs a figure we do not reproduce; it is described in words above. Lumi analyses 108 of the 174 JEE Main shifts held from 2017 to 2026.