Two swimmers, chris and sarah, start together at the same point on the bank of a wide stream that flows with a speed v. both move at the same speed c (where c > v) relative to the water. chris swims downstream a distance l and then upstream the same distance. sarah swims so that her motion relative to the earth is perpendicular to the banks of the stream. she swims the distance l and then back the same distance, with both swimmers returning to the starting point.
in terms of l c and v find the time intervals required
a) for chris's round trip and
b) for sarah's round trip.
c) explain which swimmer returns first.