**Rolling**

Imagine that you are watching the wheel of a bicycle, which passes you at constant speed while rolling smoothly (that is, without sliding) along a street. As shown in , the center of mass 0 of the wheel moves forward at constant speed .The point P on the street where the wheel makes contact also moves forward at speed so that it always remains directly below .

During a time interval r, you see both 0 and P move forward by a distance s. The bicycle rider sees the wheel rotate through an angle 0 about the center of the wheel, with the point of the wheel that was touching the street at the beginning of r moving through arc length s. Equation 11·17 relates the arc length 5 to the rotation angle.

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