Train A with proper length L moves eastward at speed v, while train B with proper length 2L moves westward also at speed v. How much time does it take for the trains to pass each other (defined as the time between the fronts coinciding and the backs coinciding):a. In A’s frame? b. In B’s frame? c. In the ground frame? d. Verify that the invariant interval is the same in all three frames.

Which of the following correctly describes what happens when an atomic bomb explodes? small pieces of fissionable material are joined and form a body with a mass greater than the critical mass, the relative number of neutrons escaping decreases, and a chain reaction and explosion result. large pieces of fissionable matter are brought together quickly and form a body with a mass smaller than the critical mass, the relative number of escaping neutrons increases, and a chain reaction and explosion result.