Internal force: the interaction force between the particles inside the system.

External force: The force exerted by the outside of the system on the internal particles of the system of particles.

The center of mass of the mass point system is referred to as the center of mass. A dimension with length that describes the position of a certain point in space related to the system of particles.

(2) The position vector of the center of mass is related to the choice of the coordinate system, but the relative position of each particle in the relative particle system remains unchanged and is a specific position.

(5) A uniform object with symmetrical shape and its center of mass is located at the center of its geometric symmetry.

(6) The difference between the center of gravity and the center of mass: The center of mass and the center of mass are different. When the size of the object is not very large, the positions of the center of mass and the center of mass coincide.

The motion of the center of mass reflects the overall motion trend of the particle system.

(1) Momentum theorem of particle system

(2) Law of conservation of momentum

(1) Angular momentum must be specified for which fixed point.

----Particle angular momentum theorem

① Work is a scalar quantity, which can be positive or negative. ② Work is a process quantity, and work only exists when the position of an object changes.

Constant force work

The power of changing force

2. The kinetic energy theorem of a particle - the effect of work

The deformation of the object after the collision can be fully restored, and the total mechanical energy of the system before and after the collision is conserved.

The deformation of the object during the collision is completely irreversible, so that the two objects merge into one and move together. There is a mechanical energy loss in the system.

The deformation of the object after the collision is only partially restored, and the system suffers a partial loss of mechanical energy.

The work done by all external forces in a mechanical system + the work done by all internal forces = the increment of the total kinetic energy of the system

For a mechanical system, according to the particle group kinetic energy theorem

2. Work of elastic force

3. Work of gravity

Universal definition of conservative force: In any reference system, the work of a pair of conservative forces only depends on the relative positions of the interacting particles at the beginning and end, and has nothing to do with the motion path of each particle.

It can be seen that the total work done by the pair of action force and reaction force is only related to the action force and relative displacement, and has nothing to do with the respective motion of each particle.

It shows that the total work of any pair of interacting forces is independent of the reference frame. Depends only on the relative displacement of the two objects.

This energy is only related to position and is called potential energy (also called potential energy). It is a kind of potential energy, which is different from kinetic energy.