作为变质量系统示例的恒重沙漏
Hourglass of constant weight as an illustrative example of a system with a variable mass
摘要：
我们为Volker Becker和Thorsten Pöschel的文章“Hourglass of Constant Weight”中提出的问题提供了一种替代方法，该文章出现在Granular Matter 10（2008），231-232，即14年前。我们的目标是在这么长时间后回到这篇文章纯粹是教学。虽然在贝克尔和波舍尔的工作中，方程（8）的推导有一个轻微而微妙的不准确之处，我们对此进行了纠正，但我们利用这种情况只是为了对一个优秀的物理问题进行详细的教学阐述，并表明在考虑变质量系统的力学时，动量通量概念的教学实用性。
英文摘要：
We provide an alternative approach to a problem posed in the article "Hourglass of Constant Weight" by Volker Becker and Thorsten Pöschel, which appeared in Granular Matter 10 (2008), 231-232, i.e. 14 years ago. Our goal to return to this article after so much time is purely pedagogical. Although there is a slight and subtle inaccuracy in the derivation of equation (8) in the work of Becker and Pöschel, which we correct, we use this circumstance only to give a detailed pedagogical exposition of an excellent physical problem and to show the pedagogical usefulness of the concept of momentum flux when considering the mechanics of systems with variable mass.
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