A disc is undergoing pure rolling motion on a flat surface. The radius of the disc is 5cm & velocity of centre of mass is 10m/s. What is the ratio of kinetic energy as seen from the ground frame to a frame moving along the disc with a speed of 10m/s?(a) 1:1, kinetic energy is frame independant(b) 3:1(c) 2:1(d) 1:3I have been asked this question during an online interview.I'd like to ask this question from Rolling Motion topic in chapter System of Particles and Rotational Motion of Physics – Class 11

A disc is undergoing pure rolling motion on a flat surface. The radius

1.	A disc is undergoing pure rolling motion on a flat surface. The radius of the disc is 5cm & velocity of centre of mass is 10m/s. What is the ratio of kinetic energy as seen from the ground frame to a frame moving along the disc with a speed of 10m/s?(a) 1:1, kinetic energy is frame independant(b) 3:1(c) 2:1(d) 1:3I have been asked this question during an online interview.I'd like to ask this question from Rolling Motion topic in chapter System of Particles and Rotational Motion of Physics – Class 11
Answer» The CORRECT choice is (b) 3:1 Easiest explanation: The kinetic energy of a pure rolling disc as seen from the ground frame is GIVEN by:1/2MV^2 + 1/2Iw^2. I = MR^2/2w = V/R ∴ 1/2MV^2 + 1/2Iw^2 = 1/2MV^2 + 1/4MV^2 = 3/4MV^2. Kinetic energy as seen from the MOVING frame will only be the energy DUE to rotation about the disc’s axis. ∴ Kinetic energy from moving frame = 1/2Iw^2 = 1/4MV^2. ∴ Ratio = (3/4MV^2) / (1/4MV^2) = 3:1.

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