10. A skier plans to ski a smooth fixed hemisphere of radius R. He starts form rest on a curved smooth surface of height TR/4 The angle at which he leaves the hemisphere is : (a) cos Yshipma (b) cos 3 (c) cos --() (d) cos [23] 5 11. A ball attached to one end of a string swings in a vertical plane such that its magnitude of acceleration at point A (extreme position) is equal to its magnitude of acceleration at point B (mean position). The angle is D (a) cos 25 (b) cos (c) cos (d) None of these