对变质量一类力学题的辨析

Analysis about a kind of the mechanical problems of the variable mass particle systems

链条或绳子下落或提升问题是变质量一类力学题的典型问题,在求解这一类问题时,用质点系的动量定理和用机械能守恒定律或质点系的动能定理常得到不同的答案.用变质量质点系的动能定理分析了以上问题产生的原因,并给出了解决的方案.在此基础上,就链条或绳子下落或提升问题的教学提出了若干建议.

The falling or raising of a chain or a rope are typical problems of the variable mass system problems.In solving this kind of problems,different answers are often given by using the momentum theorem of the particle system and the law of conservation of mechanical energy or the theorem of kinetic energy of the particle system.The reason that caused the problems is analyzed by using the theorem of kinetic energy of the variable mass particle system,and the solution is given.On the basis of the analysis,some suggestions on the teaching of the falling or raising of a chain or a rope contents are put forward.