Question

The speed of motor launch w.r.t still water is 8 m/s and the speed of stream is 4 m/s. When the launch begins travelling upstream a float was dropped from it. The launch travelled 4.8 km upstream, turned about and caught up with the float.
In this qs for upstream motion of motor launch, why do we take the relative velocity of motor launch w.r.t water as Vm - Vw = 8-4 = 4 m/s instead of 8-(-4) = 12 m/s in order to find how long is it before the launch reaches the float?
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Answer 1

Speed speed of boat with respect to water is of 12 m/s
but we are seeing it with respect to ground so in this case is boat is approaching 8m/s then water is carrying boat 4 m/ back
so resultant speed of boat with respect to ground will be 8-4= 4m/s.