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Mass is not Conserved: Instructor's Guide

Main Ideas

  1. Mass is energy.
  2. 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



  • Ask several students to present/defend their responses.
  • See also the discussion in the text.

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