I have been trying to come up with a solution for the matter but I failed, so here are my questions. I would appreciate if someone explained thoroughly. a) There is a pendulum which has a period of 20 seconds on Earth. If it was put on a vehicle moving at 0,6c and have its period measured on it, what would be the result? a.1) What if the observer was on Earth and the pendulum on the vehicle? b) This question is similiar to the first one but in reverse: The pendulum has a period of 20 seconds on the vehicle moving at 0,6c. What would be its period on Earth? b.1) What if the observer was on the vehicle and the pendulum on Earth? Thanks in advance.