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Saturday, 31 October

JEE Main 2021 · PhysicsMultiple choiceSingle correctMediumApplication

JEE Main 4 August 2021, Shift 1, Physics Q13

Question 13 of 90 in this shift, Physics question 13 of 30, Section A.

Choose the correct option for the centre of mass of a uniform disc of radius ‘R’ from which a circular section of radius r (2r < R) has been removed. The centre of the hole is at a distance ‘a’ from the centre of the disc. [Take the centre of the whole disc as the origin]
  1. (1)ar2(R2−r2)\frac{ar^2}{(R^2-r^2)}Official answer
  2. (2)arR−r\frac{ar}{R-r}
  3. (3)arR+r\frac{ar}{R+r}
  4. (4)ar2(R2+r2)\frac{ar^2}{(R^2+r^2)}

Official answer

Option 1

NTA final key.

Chapter
System of Particles and Rotational Motion
Topic
Center of Mass
Idea tested
COM of Cavity Bodies

Same idea in other shifts

Asked 3× in all
  1. 8 Jan 2020, Shift 2 · Q4As shown in fig. when a spherical cavity (centred at O) of radius 1 is cut out of a uniform sphere of radius R (centred at C), the centre…HardDiagram basedHas a figure
  2. 2 Sep 2020, Shift 2 · Q22A square shaped hole of side l=a2l = \frac{a}{2} is carved out at a distance d=a2d = \frac{a}{2} from the centre ‘O’ of a uniform circular…MediumNumerical valueHas a figure

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. Lumi analyses 108 of the 174 JEE Main shifts held from 2017 to 2026.