什么是 Reference frame; position; trajectory？

A reference frame is a rigid body taken as fixed (the laboratory, the Earth, a train), together with a clock. A point particle $M$ — a body whose size is irrelevant to the motion studied — is located at each instant by its position vector $\vect{OM}(t)$ from an origin $O$ attached to the frame; the curve described by $M$ is its trajectory. Motion is always motion relative to a frame: the same passenger is at rest in the train and moving in the station.