1. Mass is energy.
2. Energy is conserved, but mass alone is not.

Estimated Time: 10 minutes, including wrap-up

Groups are asked to analyze the following standard problem:

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.

• basic understanding of relativistic mechanics

• Table top whiteboards and markers
• Handout

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