The value of α for which angular momentum about origin is conserved is

• -1

• 2

• zero

• 1

A bar magnet having a magnetic moment of 2 x 10⁴ JT^-1 is free to rotate in a horizontal plane. A horizontal magnetic field B = 6 x 10^-4 T exists in the space. The work  is done in taking the magnet slowly from a direction parallel to the field to a direction 60^o from the field is

• 0.6 J

• 12 J

• 6 J

• 2 J

A body of mass M hits normally a rigid wall with velocity v and bounces back with the same velocity. The impulse experienced by the body is 

• 1.5 Mv

• 2 Mv

• zero

• Mv

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 

• 
• 
• 
• 

D.

Loss of energy, 

A circular platform is mounted on the frictionless vertical axle. Its radius R =2 m and its moment of inertia about the axle is 200 kg m². It is initially at rest. A 50 kg man stands on the edge of the platform and begins to walk along the edge at the speed of 1 ms^-1 relative to the ground. Time taken by the man to complete one revolution is

• π sec

• 3π/2 sec

• 2π sec

• π/2 sec