The speed of a motor launch with respect to still water is 7m/sec and the speed of stream is 3m/sec . When the launch begain travelling upstream a float was dropped from it . The launch travelled 4.2km upstream turn about and caught up the float . How long it is before the launch reaches the float .

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Q. 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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