Generally, collisions aren't head-on, but more of a sideswipe, so there's some sideways motion.

一般来说，碰撞不是正面的，而是侧面的擦碰，所以会有一些侧向运动。

And when they draw their feet out of water, they fold them up into a sideways motion.

当他们把脚从水里拉出来时， 他们会把脚折叠成横向运动。

That gives it a little bit of sideways motion, so instead of hitting the ground at my feet, it hits a bit farther away.

这就给了它一点侧向运动，所以它不会落在我脚下，而是落在更远一点的地方。

And knowing where my feet are and the strides I'm taking, we can use software to control the floor to undo my forward motion or my sideways motion.

知道我的脚在哪儿和步伐后，我们可以使用软件来控制地板，抵消我向前或侧向的运动。

Ignoring things like the Earth's rotation (which adds a bit of sideways motion) it's close enough to say the rock just falls straight down, and is stopped because the Earth itself gets in the way.

如果不考虑地球的自转的话(考虑自转会加入一点侧向运动)，这足以说明石头是垂直下落的，它会停下仅仅是因为地球本身挡住了它的去路。