31. Choose the correct statement for centre of mass reference frame: (1) The momentum of system of particles is always zero in this frame. (2) The momentum of the system of particles can never be zero in this frame. (3) The momentum of the system of particles is never conserved in this frame. (4) The momentum of the system of particles may or may not be zero in this frame. 32. A uniform disc of radius 'a' and mass 'm' is rotating freely with angular velocity in a horizontal plane, about a fixed vertical axis through its centre. A particle of mass m is then suddenly attached to the rim of the disc and rotates with it. The new angular speed is 00 (2) (1) 3 (3)5 32 33 (4) 854 33. Young's modulus Y, bulk modulus K and modulus of rigidity n are related as (1) Y (+1)= (2) Y(+) 3K (3) Y+=3 (4) Y(+)= 3 34. Theoretically the value of Poisson's " ratio () lies between (1) 0 and 0.7 (2) -1 and +1 (3) -1 and co (4) -1 and +0.5 35. The average energy of a damped. harmonic oscillator is E and its relaxation time is t. Then average power of oscillator is (1) @e E (3) 2TE (2) TE 227 (4)