方向就是轨道的切线方向。
海尔兄:沿着旋涡的切线方向。全速冲出!
EB: We are rushing out on the tangent of the whirlpool. Full speed!
这里,△r应该是近似等于。。。,它的方向是轨道的切线方向。
And now, delta r should be essentially roughly equal to, well, its direction will be tangent to the trajectory.
The tangent's pointing towards the direction you are headed at that instant.
切线的方向就是,速度矢量在那个瞬时的指向
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