A small mass attached to a string rotats on frictionless table top as shown. If the tension is the string is increased by pulling the string causing the radius of the circular  motion to decrease by a factor of 2, the kinetic energy of the mass will

• remain constant

• increase by a factor of 2

• increase by a factor 4 

• increase by a factor 4 

From a circular disc of radius R and mass 9M, a small disc of mass M and radius R/3 is removed concentrically. The moment of inertia of the remaining disc about an axis perpendicular to the plane of the disc and passing through its centre is

• 40MR²/9

• MR²

• 4 MR²

• 4 MR²

A coil has resistance 30 O and inductive reactance 20 O at 50 Hz frequency. If an AC source 200 V, 100 Hz, is connected across the coil, the current in the coil will be

• 4.0 A

• 8.0 A

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• 

A circular disk of moment of inertia I_t is rotating in a horizontal plane, about its symmetry axis, with a constant angular speed. Another disk of moment of inertia I_b is dropped coaxially onto the rotating disk. Initially, the second disk has zero angular speed. Eventually, both the disks rotate with constant angular speed.The energy lost by the initially rotating disc due to friction is 

A gramophone record is revolving with an angular velocity ω. A coin is placed at a distance r from the centre of the record. The static coefficient of friction is μ. The coin will revolve with the record if

• r = μgω²

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