| f(s) - f(t) | = | s - t | | s + t |
Can you see that if s = t + and if t is sufficiently large (depending on the undetermined ) then no matter what is chosen, | f(s) - f(t) | > 1. That would prove that the function is no longer uniformly continuous. The details are left as an exercise.
Note that this argument no longer works on a bounded interval [0, N]. Here we can not make t 'sufficiently large', since it can be no larger than N. And indeed, the function is uniformly continuous on those bounded intervals. Later we will show that any function that is continuous on a compact set is necessarily uniformly continuous.