What is Special Relativity (1)

A swimmer is swimming in the river. He wants to swim upstream from point A to point B, then swim back.

If his speed in still water is c, and the speed of the river water is v, what is the average speed of his round-trip?

Let s be the distance between A and B, then

Time from A to B:

Time from B to A:

Total Time：

Average Speed:

So the average speed is less than c.