V=2@Rcos2@t ax=-4@2Rsin2ot =-402x Vy =-2@Rsin2@t 4,=-402Rc0s20t=-4o2y x Rsin2@t :.a=va'x+a'y y Rcos2@t B =402R ↑y a=axi+ayj M =-402(ci+0) =-402r X 由此可见，的方向与矢量相反， 由M指向O点。ax = -42Rsin2t ay = -42Rcos2t vx = 2 Rcos2t vy = -2 Rsin2t x = Rsin2t y = Rcos2t y 2 x 2 a = a + a = 42R = - 42x = - 42y a = ax i + ay j = - 42 (xi + yj) = - 42 r 由此可见，a 的方向与矢量相反， 由M指向O点。 R M A O  B 2 x y