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Can you solve the 'rock in boat' puzzle?

Can you solve the 'rock in boat' puzzle?

(4:51) You throw a rock into the water from your boat. What happens to the water level? Does it rise, lower, or stay the same? youtube.com/user/physicswoman

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Guest: one more time (748 days ago)

Ha! I got the boat and rock right - using Archimedes.

I have had countless after-dinner discussions on this question: if you have a lake with water flowing in at one end and out at the other, is the lake surface level? Seems obvious to me that it is not, but I don't think I've found anyone to agree with me. One way to look at it is to think of the lake as just a wide bit of river. No-one argues that a river is level - so why should widening it change that? At what point in the widening does it become level?

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Ha! I got the boat and rock right - using Archimedes.

I have had countless after-dinner discussions on this question: if you have a lake with water flowing in at one end and out at the other, is the lake surface level? Seems obvious to me that it is not, but I don't think I've found anyone to agree with me. One way to look at it is to think of the lake as just a wide bit of river. No-one argues that a river is level - so why should widening it change that? At what point in the widening does it become level?

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TheBob TheBob (748 days ago)

So when you take the plug out of the bath (with a special mechanism that avoids disturbing the surface) - are you saying the water level dips down towards the plug end?

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So when you take the plug out of the bath (with a special mechanism that avoids disturbing the surface) - are you saying the water level dips down towards the plug end?

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Guest: one more time (747 days ago)

OK, it all depends on how accurate you want to make the model. For instance, we are assuming a flat world and ignoring the curvature of the earth in the bathtub - but that is a simplification. Anyway, when you take the plug out, water starts to drain, which pulls in water beside it, and so on. It takes time for that disturbance to propagate from one end of the bath to the other, presumably at something ike the speed of sound in water. Google says that is about 1500 m/s so in a 2 metre bath (I wish they were that big!) it will take 2/1500 s = a millisecond or thereabouts. That millisecond gives the time for the plug end to drain more than the far end.

Also there is viscosity. If it was liquid honey in the bath, not water, you would not be surprised to see the surface of the liquid being funnel shaped towards the plug. Water is a lot less viscous than honey but it will still have some effect.

Now all we need is a real physicist (not a pretend one like me) to offer to calculate how much slope the bath tub will actually have.

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OK, it all depends on how accurate you want to make the model. For instance, we are assuming a flat world and ignoring the curvature of the earth in the bathtub - but that is a simplification. Anyway, when you take the plug out, water starts to drain, which pulls in water beside it, and so on. It takes time for that disturbance to propagate from one end of the bath to the other, presumably at something ike the speed of sound in water. Google says that is about 1500 m/s so in a 2 metre bath (I wish they were that big!) it will take 2/1500 s = a millisecond or thereabouts. That millisecond gives the time for the plug end to drain more than the far end.

Also there is viscosity. If it was liquid honey in the bath, not water, you would not be surprised to see the surface of the liquid being funnel shaped towards the plug. Water is a lot less viscous than honey but it will still have some effect.

Now all we need is a real physicist (not a pretend one like me) to offer to calculate how much slope the bath tub will actually have.

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Guest: Just a thought (747 days ago)
Latest comment:

A current, in a river, is the flow of water influenced by gravity and not necesarally levels. The current varies within the river are dependent upon the flow volume of water and channel geometrics (levels).

Tidal currents are a result of the rise and fall of the water level due to tides (regardless of levels).

Levels are influenced by many factors. In other words the surface will remain level regardless of the bottom of the river.

If there's a dip in the river bed there wont be a sudden dip in the water level on the surface.

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Original comment
Latest comment:

A current, in a river, is the flow of water influenced by gravity and not necesarally levels. The current varies within the river are dependent upon the flow volume of water and channel geometrics (levels).

Tidal currents are a result of the rise and fall of the water level due to tides (regardless of levels).

Levels are influenced by many factors. In other words the surface will remain level regardless of the bottom of the river.

If there's a dip in the river bed there wont be a sudden dip in the water level on the surface.

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