Dynarnics System of free particles: a system in which the motion of each particle is not restricted by any constraints System of non-free particles refers to a system in which the motion of particle is restricted by constraints A system of particles is the most general abstract model of mechanics including rigid bodies, elastic bodies and fluids Particles dynamics Particle dynamics is the basis of the Dynamics of a system of particles dynamics of a system of particles 4. The fundamental problems of dynamics There are two cases in general. One is to determine the acting forces when the condition of motion is known, the other is to determine the motion of the body when the acting forces are known There are still some additional comprehensive problems. Sometimes part of the forces and part of the motion are known, we have to determine the other part In some cases, the positive forces are known, we should determine the motion firstly, then determine the reaction forces of constraints5 System of free particles: a system in which the motion of each particle is not restricted by any constraints. System of non-free particles refers to a system in which the motion of particle is restricted by constraints. A system of particles is the most general abstract model of mechanics including rigid bodies, elastic bodies and fluids. 3. Classification of dynamics: Particles dynamics Particle dynamics is the basis of the Dynamics of a system of particles dynamics of a system of particles 4. The fundamental problems of dynamics: There are two cases in general. One is to determine the acting forces when the condition of motion is known, the other is to determine the motion of the body when the acting forces are known. There are still some additional comprehensive problems. Sometimes part of the forces and part of the motion are known, we have to determine the other part. In some cases, the positive forces are known, we should determine the motion firstly, then determine the reaction forces of constraints.   