This small group activity is designed to help upper division undergraduate students work out the lifetime of cosmic rays taking relativistic effects into account. Students work in small groups and use hyperbolic trigonometry and spacetime diagrams to calculate the lifetime of incoming cosmic rays. The entire class wrap-up discussion summarizes group findings and difficulties, helping students gain a geometric perspective of time dilation and length contraction.

This material is part of the Paradigms in Physics project at Oregon State University. This work promotes the use of active student learning in upper division physics courses. Both learning materials and learning strategies are provided to help both students and instructors.