Thecla said:
Over the years I've tried...
1) "If we graph the kinetic energy of an object as a function of its speed, there's a vertical asymptote at ##c##. " This worked with a high schooler who was finishing up Algebra II.
2) "If you could get from point A to point B faster than a beam of light, then there would be someone in the universe who would see your arrival before your departure - and would be able to prevent your departure after they had seen the arrival. " Useful only with an audience that is thinking it's a technological problem, that we need more powerful motors and better spaceships.
3) "The laws of electromagnetism predict, and experiments confirm, that the speed of light is the same for everyone regardless of their relative speed..." It's easy to show that this is incompatible with a relative speed greater than ##c## by considering a flash of light trying to catch up to something traveling faster than light.
4) "No how much force we use to speed something up, the speed increase gets smaller as it gets closer to the speed of light - diminishing returns means that we can't ever quite get there". As with #1, this is the enforcement mechanism for the arguments by contradiction in #2 and #3. The velocity addition formula can be used here if your audience is willing to accept it on faith.
All of these occupy the dubious ground between lies to children, flat out untruths, and grotesque oversimplification. But they all have worked for me at least once - it helps to know your audience. And of course none of these are really answers to the question, they're more different ways of saying "because them's the rules and we have the experiments to prove it"
But seeing as how we're talking about "an average person with high school math knowledge"... I wouldn't give up on just doing it right and deriving the Lorentz transformations.
