A problem by Soviet physicist Viktor Lange:

It’s not uncommon to see two ships traveling down a river at different velocities — this is due to differences in design and engine power.

“But why can rafts which have no engines float down the river with different velocities, too? It has been even noticed that the heavier the raft the higher its velocity. Why is this?”

| SelectClick for Answer | | --- | | The water in a river doesn’t move with a single velocity — friction with the air and the channel slows some of the stream. Water moves fastest just below the surface in midstream:As a raft is laden its draft increases, submerging it into the faster layers, which carry it along more quickly.From Physical Paradoxes and Sophisms, 1987. |

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