Hello everyone, This is not a typical homework question. I found this confuses me when my teacher derives the special relativity formula in the class. Here is the derivation: 1. The astronaut, stationary relative to the clock incorporated in the bottom mirror, measures a time interval 2 delta t(s) for the light to make a round trip and so infers delta t(s) to be the duration of the upward journey. 2. The time interval as measured by an observer on Earth, who records it as delta t(m). 3.The path of the light forms two identical right triangles So, the textbook and my teacher use the Pythagorean theorem to make a equation, and isolate delta t(m). Since the scenario was testing with the path of light, therefore the formula would include c square, of course. Now, my question is: Can we use this formula for the other problems that are testing with the path of things other than light, such as a ball. If we can, should not we replace the speed of light with the speed of the ball? I did talk to my teacher about it, but he could not find any answers yet, so I guess I may get some help here. Sorry about my grammar. I hope I state my question clear. Thanks to anyone who are trying to help!