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How does the width or depth of a container of water affect the speed of an object (e.g. a toy boat) travelling on it?

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Try thinking along these lines:

When the object moves forwards, water has to move backwards, to make a new "hole in the water" to be occupied by the object, and to fill up the "hole" where it was before.

If there is only a narrow space between the object and the walls, the water has to move backwards faster than the object is moving forwards.

You might want to try googling for "virtual mass" in fluid flow, which is another way of thinking about the same effect.

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Look up laminar flow and notice that resistance increases with velocity gradient (difference in velocity over separation). The further away the walls the lower the velocity gradient (the higher the separation).

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  • $\begingroup$ @grace - the question says "an object travelling in water" and I take that to mean an object immersed in water, not above it. You can edit the question to make it clearer. $\endgroup$ – John Alexiou Jun 24 '20 at 5:27

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