A, reference and particle in order to study the motion of the object and assume that the object is fixed, known as the reference. When studying the motion of an object, the size and shape of the object are not considered, but the object is regarded as a point with mass. This point where mass replaces an object is called a particle. 1, the necessity of selecting a reference object The change of the position of an object relative to other objects is called mechanical motion, which is referred to as motion for short. Mechanical movement is the most common natural phenomenon, and all objects in the universe are constantly moving. Therefore, when we study the motion of an object, we must assume that an object is stationary and refer to this object to determine the motion of other objects. 2. How to choose the reference for the same action? Because the selected reference objects are different, the observation results are often different. For example, if a passenger sitting on a moving train chooses a carriage as a reference, then the passenger is stationary relative to the carriage; If you choose the tree next to the railway as a reference, the passengers move with the train. The choice of reference is often for the convenience of studying problems. When studying the motion of objects on the ground, the earth is often used as a reference; When studying the motion of planets in the solar system, the sun is the most suitable reference, that is, assuming that the sun is stationary. 3. Particle is a scientific abstraction. Physics simplifies practical problems and is called scientific abstraction. Scientific abstraction is not arbitrary, but must proceed from practical problems. For example, when we study the revolution of the earth, because the diameter of the earth (about 1.3× 10 4 km) is much smaller than the distance from the earth to the sun (1.5× 10 8 km), we can ignore the size and shape of the earth, that is, treat the earth as a particle. But when studying the earth's rotation, we can't ignore the size and shape of the earth, and we can't regard the earth as a particle. Generally speaking, when studying the motion of objects on the earth, unless the rotation of objects is designed, objects can be regarded as particles. The following example in motion can be regarded as a particle (). A, figure skater B, ocean-going ship C, rotating grinding wheel D, block of wood sliding down the slope. Solving particles is a scientific abstract concept in mechanics and an idealized model. When studying some problems, if the size and shape of an object play a negligible role in the phenomenon studied, it can be regarded as a particle. Athletes who do figure skating have movements that can't be ignored, such as rotation, and the movements of different parts of the body are not the same, so they can't be regarded as particles. In the process of rotation, the size and shape of the grinding wheel play a major role in the movement and cannot be ignored, so it cannot be regarded as a particle. The great ship in ocean voyage is very small relative to the relevant distance, and the movement of each point of the block sliding down the inclined plane is the same, which can be regarded as a particle. So choose b and d.