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## Introduction to Angular Momentum Lecture ( minutes)

Central Forces Notes Section 6

- Small whiteboard question: Write down something about angular momentum. Students are much more likely to remember $L = I \omega$, the specific formula for simple rigid body rotation, than the general definition, $\Vec L = \Vec{r} \times \Vec{p}$.
- Begin with the idea that angular momentum is defined about a specific point. Always say the angular momentum of $x$ about the point $y$.
- Often it is useful to include a small whiteboard question about the cross product to refresh students memories about the meaning/interpretation of the cross product in the equation $$ \Vec{L} = \Vec{r} \times \Vec{p}$$.
- It is often useful to discuss the angular momentum for a particle moving in a straight line around a point off that line.
- Derivation of the expression $$ {d{\Vec{L}}\over{d{t}}} = \Vec{r} \times \Vec{f} = \Vec{\tau}$$
- Given a central force, $\Vec{\tau} = 0$ and $ {d{\Vec{L}}\over{d{t}}}$ is conserved.
- Since angular momentum is conserved, the motion is in a plane.