保持直线飞行，包括高速的直线飞行，都不需要任何净外力。

To keep us travelling in a straight line, even at high speed, requires no nett force.

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因此，炮弹由直线运动变成高速的环形运动，现在正沿一条椭圆形轨道绕着月亮飞行，变成了一颗真正意义上的卫星。

The rectilinear movement has thus become changed into a circular motion of extreme velocity, and it is now pursuing an elliptical orbit round the moon, of which it has become a true satellite.

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