Learn

Saturday, 31 October

JEE Main 2021 · PhysicsMultiple choiceSingle correctEasyApplication

JEE Main 27 August 2021, Shift 2, Physics Q3

Question 3 of 90 in this shift, Physics question 3 of 30, Section A.

Two discs have moments of intertia I1I_1 and I2I_2 about their respective axes perpendicular to the plane and passing through the centre. They are rotating with angular speeds, ω1\omega_1 and ω2\omega_2 respectively and are brought into contact face to face with their axes of rotation coaxial. The loss in kinetic energy of the system in the process is given by :
  1. (1)I1I22(I1+I2)(ω1−ω2)2\frac{I_1I_2}{2(I_1+I_2)}(\omega_1-\omega_2)^2Official answer
  2. (2)I1I2(I1+I2)(ω1−ω2)2\frac{I_1I_2}{(I_1+I_2)}(\omega_1-\omega_2)^2
  3. (3)(ω1−ω2)22(I1+I2)\frac{(\omega_1-\omega_2)^2}{2(I_1+I_2)}
  4. (4)(I1−I2)2ω1ω22(I1+I2)\frac{(I_1-I_2)^2\omega_1\omega_2}{2(I_1+I_2)}

Official answer

Option 1

NTA final key.

Chapter
System of Particles and Rotational Motion
Topic
Angular Momentum
Idea tested
Kinetic Energy Loss in Rotational Coupling

Same idea in other shifts

Asked 5× in all
  1. 30 Jan 2024, Shift 1 · Q52Consider a Disc of mass 5 kg, radius 2 m, rotating with angular velocity of 10 rad/s about an axis perpendicular to the plane of rotation.…EasyNumerical valueHas a figure
  2. 30 Jan 2024, Shift 2 · Q52Two discs of moment of inertia I1=4 kg m2I_1 = 4\ kg\,m^2 and I2=2 kg m2I_2 = 2\ kg\,m^2, about their central axes & normal to their planes, rotating with…EasyNumerical value
  3. 2 Sep 2020, Shift 2 · Q4Two uniform circular discs are rotating independently in the same direction around their common axis passing through their centres. The…EasySingle correct
  4. 4 Sep 2020, Shift 1 · Q22A circular disc of mass M and radius R is rotating about its axis with angular speed ω1\omega_1. If another stationary disc having radius…MediumNumerical value

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.