原题如图1,小球套在光滑的竖直杆上,轻弹簧一端固定于O点,另一端与小球相连。现将小球从M点由静止释放,它在下降的过程中经过了N点。已知M、N两点处,弹簧对小球的弹力大小相等,且∠ONM<∠OMN<π/2。在小球从M点运动到N点的过程中()A.弹力对小球先做正功后做负功B.有两个时刻小球的加速度等于重力加速度C.弹簧长度最短时,弹力对小球做功的功率为零 The original title shown in Figure 1, the ball sets a smooth vertical rod, one end of the light spring fixed at the O point, the other end connected with the ball. Now the ball is released from rest at point M, which passes through point N as it descends. Known M, N two points, the elastic force of the spring on the ball equal size, and ∠ ONM <∠ OMN <π / 2. In the process of moving the ball from point M to point N () A. The elastic force does negative work on the ball after doing positive work B. There are two moments when the acceleration of the ball is equal to the acceleration of gravity C. When the spring length is the shortest, the elastic force The power to work on the ball is zero