如图所示的荡木机构中，O1O2 = CD，O1C = O2D = 1m，在图示位置时O1C的角速度为 1rad/s，角加速度为<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/6944cb8be2fe4f58b114e0ddd55ef12d.png?x-oss-process=image/sharpen,200"/>，则荡木中点M的加速度为（）<br/> <img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/475ae05b4c1b470ca19350559eaa8ff4.png?x-oss-process=image/sharpen,200"/>