move back

1.She unconsciously moved b<>an class="key">ack a pace or two.

她下意识地向后退一两步。

2.The policeman directed the crowd to move back for safety.

全那个警察命令人群往后退。

3.The process beginnng from powder burning to the ejection pistons pushing out the bomb and its moving back was analyzed applying classical interior ballistics and gasdynamics theory.

给出对某型挂弹钩的实例计算，计算结果与试验结果基。