液体流、电子流、光子流、气体流可广义地视为“流体”,其中电子流、光子流、气体流可统称为粒子流．对于液体流,可选取质量为△m的液柱为研究对象;对于粒子流,通常是以体积元△V=△t·v·S内的粒子为研究对象,这样就可以用动量定理、动能定理来处理问题． Liquid flow, electron flow, photon flow, and gas flow can be broadly viewed as “fluids,” where electron flow, photon flow, and gas flow can be collectively referred to as particle flow. For the liquid flow, the liquid column with the mass Δm can be selected as the research object; for the particle flow, the particles in the volume element ΔV=Δt·v·S are usually used as the research object, so that the momentum theorem and kinetic energy can be used. Theorem to deal with the problem.