A motorboat going downstream overtakes a float at point M. Sixty minutes later it turns back and later passes the float at a point 6 km from M. Find the stream speed, assuming constant motorboat speed relative to still water.
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Question

A motorboat going downstream overtakes a float at point M. Sixty minutes later it turns back and later passes the float at a point 6 km from M. Find the stream speed, assuming constant motorboat speed relative to still water.

Options

G1 Physics · Accepted Answer

The float moves only with the stream. From the first meeting to the second meeting, the elapsed time is 2 h: one hour until turn and one hour for the relative return because the boat's speed relative to the float is the same as its speed relative to water. The float travels 6 km in 2 h, so stream speed=3 km h⁻¹. Option C.