backward和backwards有什么区别

　　The car passed over the body twice, once backward and then forward.

　　那辆车把那人来回轧了两次，先是倒着轧的，再是往前开着轧的。

　　He took two steps backward.

　　他后退了两步。

　　She was rocking backwards and forwards in her seat.

　　她在座位上前摇后晃。

　　I punched him on the chin, sending him reeling backwards.

　　我一拳击中他的下巴，打得他向后打了个趔趄。