太阳系的自由落球

2021年8月25日

Solar System Ball Drop
Video Credit & Copyright: James O’Donoghue (JAXA) & Rami Mandow (Space Australia); Text: James O’Donoghue

Explanation: Does a ball drop faster on Earth, Jupiter, or Uranus? The featured animation shows a ball dropping from one kilometer high toward the surfaces of famous solar system bodies, assuming no air resistance. The force of gravity depends on the mass of the attracting object, with higher masses pulling down with greater forces. But gravitational force also depends on distance from the center of gravity, with shorter distances causing the ball to drop faster. Combining both mass and distance, it might be surprising to see that Uranus pulls the ball down slightly slower than Earth, despite containing over 14 times more mass. This happens because Uranus has a much lower density, which puts its cloud tops further away from its center of mass. Although the falling ball always speeds up, if you were on the ball you would not feel this acceleration because you would be in free-fall. Of the three planets mentioned, the video demonstrates a ball drops even faster on Jupiter than either Earth and Uranus.

Tomorrow’s picture: open space

太阳系的自由落球
影片提供与版权: James O’Donoghue (JAXA) & Rami Mandow (Space Australia); 文稿: James O’Donoghue

说明: 释球于地球、木星或天王星，在何者的落速较高？这部主题动画，呈现在多个太阳系著名天体的表面上方1公里高之处把球释放，在忽略空气阻力的条件下，球自由下降的状况。落体（球）受到的引力与受力的物体之质量有关，质量愈大，引力愈大。此外，物体受到的引力也和它与引力中心的距离有关，间距愈小，球（物体）的落速愈高。如果同时考量质量和距离，读者或许会有点惊讶，球在天王星的落速比在地球稍慢一些，緃然天王星的质量是地球的14倍之多。原因是天王星的密度远低于地球，因此它的云顶离质量中心颇远。虽然球的落速恒增，不过如果你站在球上，处于自由落体状态的你，感受不到加速。在上述的这三个行星之中，这部影片证实球在木星的落速，高于在地球和天王星。

(注：Sun 太阳、Mercury 水星、Venus 金星、Earth 地球、Moon 月球、Mars 火星、Ceres 谷神星、Jupiter 木星、Saturn 土星、Uranus 天王星、Neptune 海王星、Pluto 冥王星)

明日的图片: open space