Hello, I've read and learnt the fundamentals of SR and understand it to quite a nice extent. I'm talking about the time dilation's derivation using a light clock. I've understood the derivation's concepts, i.e. the proved principle of speed of light being 'c' in all frames of reference. So that part is clear. Now lets say the light beam is replaced by a soccer ball, which also oscillates the same way except the fact that it does so once in every one second (Just to keep things practical). Now, I wish to calculate the time dilation (though it would be extremely small). Now, I'm not sure if just like light, the balls speed would appear to be the same to the external observer. If yes, then can someone elaborate on this? In that case, what would have classical mechanics predicted? Thanks for your time, Sleek.