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=====Position Vector Lecture/Discussion (15 minutes)=====

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{{courses:lecture:cflec:central_forces_notes.pdf|Central Forces Notes}} Section 7

  * Review of polar coordinates and how to graph an equation in polar coordinates
  * Students are given the following diagram and asked to write down the position vector in polar coordinates on their small white boards
  * Students are sometimes confused about the ellipse drawn on the board. They often think it is a 3D perspective drawing of a circle rather than an ellipse drawn in the plane of the board. It is important to clarify this.
  * Some useful questions to get the students thinking about the differences between  $\Vec {r}$, $\hat {r}$ and  $\hat {\phi}$ are: 
    - Where is the tail of the position vector $\Vec {r}$  anchored? 
    - What about $\hat {r}$ and  $\hat {\phi}$? 

  * It is often helpful to students if we emphasize that $\Vec {r}$ always points from the origin, but that  $\hat {r}$ and  $\hat {\phi}$ have their tails at the tip of the position vector $\Vec {r}$.

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