Question

A stream with current running at 4km/h is 250 m wide between two parallel banks .A boat has to cross the stream. It's speed in still water is 6 km/h .
1. In what direction will it cross the stream in the least possible time.?
2.In what direction will it cross the stream at a point directly opposite its starting point.
3.calculate the time travelled in each case and the distance moved downstream in (1).

Answer 1

Draw a diagram.
#1. It heads directly across the stream, but will move downstream during the crossing. Crossing speed: 6 km/hr
#2. To move directly across, it has to cross heading upstream at an angle θ, where sinθ = 4/6. Crossing speed: 6cosθ = √(6^2-4^2)
#3. In each case, time = distance/speed (where speed is, of course, in m/s)