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Central Forces Notes Section 6

• Small whiteboard question: Write down something about angular momentum.
• 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.
• 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.