Ariver has a steady speed of 0.500 m/s. a student swims upstream a distance of 1.00 km and swims back to the starting point. (a) if the student can swim at a speed of 1.20 m/s in still water, how long does the trip take? (b) how much time is required in still water for the same length swim? (c) intuitively, why does the swim take lon- ger when there is a current?

A. Upstream refers to the motion of the swimmer where he is against the current. The resultant speed of the swimmer is equal to the difference of the velocity or speed in still water and that of the river. The time it requires to cover the distance is calculated through the equation,
               t = d / s
where t is time, d is distance, and s is speed. Substituting the known values,
             t = 1000 m / (1.2 m/s - 0.5 m/s) = 1,428.57 seconds

(b) The time it requires for the swimmer to swim in still water,
            t = 1000 m / (1.2 m/s) = 833.33 seconds

(c) Intuitively, it takes longer to cover the distance when there is current because the current will serve as resistance to the motion of the swimmer, partially moving it backwards instead of forward. 

A) acceleration. this seems to be the one that makes sense.