Since Brice is at rest in the train, the speed of Brice relative to Igor is v′B(t) (the speed of the train, or the speed of the x frame of reference relative to the x′ frame of refer...Since Brice is at rest in the train, the speed of Brice relative to Igor is v′B(t) (the speed of the train, or the speed of the x frame of reference relative to the x′ frame of reference). which tells us how to obtain the position of object A in the x′ reference frame, when xA(t) is the description the object’s position in the x reference frame which is moving with a velocity v′B relative to the x′ reference frame.
