P. 3898. A horizontal disc of mass m=0,6 kg and radius R=10 cm can be rotated easily about a vertical axle. Along a radius of the disc a sheet is fixed to the disc. (The mass of the sheet is negligible.) A horizontally fired bullet, whose mass is m1=20 g, hits the sheet perpendicularly, and the disc begins to rotate at an angular velocity of =20 s-1. The distance between the axle and the place of impact is r=8 cm. What was the initial speed of the bullet, if a) it is embedded in the sheet; b) it bounced back totally elastically?