The force is always perpendicular to the direction of motion.

因为这个力总是垂直,于运动的方向。

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And the force is perpendicular to the arm.

力是垂直于杆。

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And then there is the normal force perpendicular to the surface.

这个力，垂直于斜面。

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And then there is the normal force perpendicular to the surface.

这个力，垂直于斜面。

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