给出变质量质点的动能定理和一对反推内力作功的表达式，并由此给出软绳问题的正确解释。
A theorem on kinetic energy for variable mass-point and formula of work of recoil internal forces are given and the reason of soft-rope query is discussed.
给出变质量质点的动能定理和一对反推内力作功的表达式，并由此给出软绳问题的正确解释。
A theorem on kinetic energy for variable mass-point and formula of work of recoil internal forces are given and the reason of soft-rope query is discussed.
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