A person swims with a speed v realative to water What dose this mean?
it means to find true speed over ground, you have to add water velocity.
Example, a river is going 10m/s North.
You jump in the water, with zero relative speed...you will go with the current, speed over ground 10 N
you swim upstream at 2m/s back to the dock, but the stream is going 10 m/s North. Your speed over ground is 8 m/s North.