# MRF's: views from different reference frames

Rolling without slipping

The observed motion of a point depends on the motion of the observer. Consider point P on the robotic arm shown below. Three GIFs are shown below the provide the view of the motion of point P as seen by three observers: i) an observer on fixed ground; ii) an observer attached to link OA; and, iii) an observer attached to link AP.

Observer on link OA
Here, the observed path of P is complicated. The observed motion includes the rotation of both links OA and AP.

Observer on link OA
Here, the observer is aware of only the rotation of AP about point A. Therefore, the observed path of P is a circle centered on A.

Observer on link AP
With the observer on the same rigid body at P, the observer is unaware that point P actually moves. Therefore, the observer sees a stationary point P. The observer also sees link OA rotating in the clockwise sense, when, in fact, it is rotating in the counter-clockwise sense.

Question: Which observer do you think would be most useful in calculating the velocity and acceleration of point P?