这是角度dθ,它运动了一会儿,运动过角度dθ,弧度是，半径是。
Let this be the angle d theta so it moves in a short amount of time over an angle d theta dS and the arc here is dS, R and the radius is R.
,这个弧度的半径，也是个空气轨道，等于115上下5米。
the radius of curvature of our arc, which is also an air track, equals 115 plus or minus five meters.
一弧度就是长度刚好等于半径的一段圆弧所对的圆心角。
A radian is the Angle subtended by an arc of a circle whose length is just equal to the radius.
一弧度就是长度刚好等于半径的一段圆弧所对的圆心角。
A radian is the Angle subtended by an arc of a circle whose length is just equal to the radius.
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