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In order to explore the evolution law of mechanical strength of frozen and thawed coal samples after the freezing of liquid nitrogen (LN 2 ) at ultralow temperature, the Yuanzhuang bitumite is cored to make standard samples. The MTS testing machine and ultrasonic detector are used for testing, and the stress−strain curve and ultrasonic time domain image of coal samples are obtained. The evolution law of mechanical strength of coal samples is studied. The results showed that (1) uniaxial compressive strength, elastic modulus, and ultrasonic longitudinal wave velocity of frozen coal samples are positively correlated with the freezing time of LN 2 in 0−50 min and negatively correlated with the freezing time of LN 2 in 50−150 min. (2) The uniaxial compressive strength, elastic modulus, and ultrasonic longitudinal wave velocity of thawed coal samples decrease with the increase of LN 2 freezing time. (3) With the increase of the LN 2 freezing time, the time-domain waveform of coal samples begins to be disordered, and the frequency-domain waveform begins to shift to low peaks, and branch peaks are generated. (4) The brittleness index of frozen coal samples increases with the increase of freezing time, and the brittleness index of thawed coal samples decreases with the increase of freezing time. Compared with the frozen coal samples, the thawed coal samples are subjected to greater thermal stress under the freezing of LN 2 at the same time, which shows a stronger deterioration of mechanical properties. The results can provide a reference for LN 2 fracturing and antireflection.

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In this essay, we believe that land value tax is efficient in the short term while being unfair in the general term. In 1935, the United States was in the midst of the Great Depression, and to make a living, Charles Darrow sold the "Monopoly game" to the toy manufacturer Parker Brothers (Pilon, 2015). The game became an overnight hit, but the real inventor behind this century-old game was Elizabeth Magie, one of the followers of George. Henry George's book Progress and Poverty deeply influenced Magie, "the right to use the land is like people breathing equally". Magie spent the rest of her life proving that George's philosophy was right. In the second mode of the game, Prosperity, which Monopoly never came out with, the lands are not being owned privately. As long as someone makes money from the trade of land, all other players can gain from it. This design reflects Magie's advocacy of land value tax, which rejects private ownership of land and promotes land as the common property of mankind.

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