两题中大推甄试题，不知道怎麽翻比较好，请教一下，感激不尽!! A rigid body is said to be translating if all lines in the body remain parallel to their original positions. Therefore, translation implies the absence of rotation. Consequently, the velocities and accelerations of all points of the body are equal, so the motion of any one point of the body determines the motion the entire body. On the other hand, rotation about a fixed axis is a special case where one line in the body, called the axis of motion, is fixed in space. Since the body is rigid, the path of each point (except points on the axis of rotation) is a circle with its center on the axis of rotation. All points remain a fixed distance from a reference plane that is perpendicular to the axis of rotation. In problems of mechanics, we can delineate different classes of situations concerning equivalence of vectors. First, we have the situations in which vectors may be positioned anywhere in space without loss or change of meaning provided that magnitude and direction are kept intact. Under such circumstances the vectors are called free vectors. On the other hand, there are situations in which the vectors must be applied at definite points. The point may be represented as the tail or head of the arrow in the graphical representation. These vectors are called bound vectors --

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