A boat, which has a speed of 5km/h in still water, crosses a river of width 1km along the shortest path possible in 15 minutes. Find the velocity of the river?
The river is moving at a speed of 3 km/h.
If the speed of the river is constant over the stretch that the boat is sailing, we can use Pythagoras:
If the river had been still, the boat would have moved from A to B, 1 km.
Instead, because the water is moving, the boat moves from A to C,
This stretch must be
According to Pythagoras,
As the river, therefore, moves