Realizing that space-time symmetries provide more intuitive introduction to the abstract notion of a symmetry used in physics, on day 17 we switched gears by discussing Richard Feynman’s lecture “Symmetry in Physical Law,” which students watched outside the class.

Feynman introduces the notion of a symmetry of a physical law, which we disected in the context of Coulomb’s law.

- We defined a
**coordinate system**and discussed the difference between**passive**and**active****transformations**. - We worked out the
**translational**and**rotational spatial symmetries**in 2D and showed that**translations form a group**. - We also introduced
**translations in time**and showed how they leave the kinematic equations unchanged.

As a transition to the discussion of moving observers, we watched the first half of a wonderful video from the 60’s titled *Frames of Reference. *