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===== Mass is not Conserved: Instructor's Guide =====

==== Main Ideas ====

  - Mass is energy.
  - Energy is conserved, but mass alone is not.

==== Students' Task ====
//Estimated Time: 10 minutes, including wrap-up//

Groups are asked to analyze the following standard problem:

== Mass is not Conserved ==

Two identical lumps of clay of (rest) mass m collide head on, with each
moving at 3/5 the speed of light.  What is the mass of the resulting lump of
clay?

==== ====

//If time permits, further problems from the handout can be considered.//

==== Prerequisite Knowledge ====

  * basic understanding of relativistic mechanics

==== Props/Equipment ====

  * [[props:start#whiteboards|Table top whiteboards]] and markers
  * {{activities:content:handouts:rfmasshand.pdf|Handout}}

==== Wrap-Up ====

  * Ask several students to present/defend their responses.
  * See also the discussion [[gsr>book:gsr:hwmass|in the text]].