Yacht problem

Sep 2019
3
0
The entrance to a harbour is a channel of length A which runs between two banks a distance B apart. The banks and the channel can be assumed to be rectangular. There is a current flowing with a speed v from one bank to the other. A yacht travelling at a constant speed relative to the water u wants to enter the harbour.

Find the minimum value of u for the yacht to safely enter the harbour. Express u in terms of v,A and B

Need help solving this. I keep getting told we cannot assume the yacht is perpendicular to the water flow
 
Jun 2016
1,151
520
England
The velocity of the yacht must be such that over the distance A the current v will push it sideways by the distance B.
Find the (equation for the) time taken for the flow v to push the yacht a distance of B
find the (equation for the) speed (along the channel) required for the yacht to move the Distance A in that time.
This gives you (the equations for) two (perpendicular) velocities
add (the equations for) these two velocities to get the (equation for the) total velocity of the yacht.
 
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