A boat in water, pressure, motion, et cetera

In summary: Originally posted by Integral In your thought experiment, ∂ρ/∂t is some value, a constant for simplicity, say Jsink. This gives you an equation for the flow of water. Now, just determine the coupling...There must be a coupling between the sink and the bucket, or the water would just pour out of the bucket. Something must be pushing the water into the bucket. I'm not sure what you are asking.
  • #1
Adam
65
1
A question (and no, it's not for uni work).

A boat sits in the water. Attached to the front of the boat is a bucket (or some other vessel), submerged. The bucket is sort 0of a black hole for water. Anything going into the bucket vanishes, to another world, the Pentagon, Michael Jackson's basement, whatever. It just goes.

Now, the water will pour into the submerged bucket due to gravity and pressure and all. That's happening at the front of the boat.

Will the boat move forward?
 
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  • #2
No, because water is entering the bucket from every direction.
 
  • #3
You pose a non physical question and want a physical answer?

With your magic bucket in the water it is anybodys guess as to what will happen. Since you made up the bucket you can make up the effect it has.
 
  • #4
Originally posted by Integral
You pose a non physical question and want a physical answer?

With your magic bucket in the water it is anybodys guess as to what will happen. Since you made up the bucket you can make up the effect it has.

The Shrodinger's Cat 'thought experiment' was hardly a physical question, yet it helped illustrate a good point.
Integral, don't you think that questions such as the one posed help us to improve our grasp of Physics?
 
  • #5
Schroedinger's cat, Ie a cat in a box, was completely physical.

This proposes vanishing water, we have no mechanism for it, it simply vanishes. Ok, make it a black hole, then its not a bucket in the water, its a black hole. Perhaps something could be done with that. We need a real world mechanism to reach real world conclusions.

The given statement is something out of sci fi, so the answer can be given from the same realm, that is anything is possible.

No, I do not think seeking physical solutions to non physical situations aids in understanding the real world. As soon as you massively violate a basic physical principle we have exited the real world for a fictional one. I prefer to deal with real world problems.

If you are writing a Sci Fi story where such a thing exists then you can also create the effects it will have.
 
  • #6
Originally posted by Adrian Baker
The Shrodinger's Cat 'thought experiment' was hardly a physical question,

Yes it was. That's exactly the point. A thought experiment consisting of nothing non-physical, can be devised, which has an ambiguous outcome. It's exactly why that experiment is such a valuable tool for understanding physics. OTOH, a bucket which simply causes water to disappear, is non-physical, or at best, an incomplete description. If it's a black hole, we can figure out what will happen. If water just disappears, we cannot. Does a vacuum exist below the not-yet-disappeared water? Does the disappearing force affect nearby water without disappearing it? Does gravity work normally near the bucket? Can the bucket really differentiate between water molecules and boat molecules? How? Unless we know these things, there is no way to calculate the rate of disappearance of water, and no way to say what will happen.

Edit: Sorry, I was typing the same time as Integral, and said mostly the same thing.
 
  • #7
If the "bucket" is afcing forward then yes, the boat should move forward. Water pressure in front of the bucket will be less than that behind it, creating hydrodynamic lift in whatever direction the open end of the bucket is facing.
 
  • #8
Okay, change "magic bucket" for "mobile drain" which is stuck to the front of the boat. It doesn't matter. Water is leaving from in front of the boat. It seems to me that would mean slightly less pressure at the front end, so pressure around the boat from everywhere else would force forward motion. Is this wrong?

Another question. Have you ever filled the kitchen sink, or a bowl, with water, then poured some washing detergent on top, and made a small vaguely boat-shaped (pointed bow, flat stern) thing and sat it on the top?
 
  • #9
Originally posted by Adam
Anything going into the bucket vanishes, ...

Now, the water will pour into the submerged bucket due to gravity and pressure and all.
Are you suggesting that there is some gravitation that only interacts with water? What it looks like your trying to do is contrive a sink for water, and then qualify the effects on the surrounding objects in the water. This is very close to a physical model, and it doesn't violate any physical laws that I know. There is a continuity equation that says:

∇*J = -∂ρ/∂t.

(The asterisk is intended to indicate the dot product).

In your thought experiment, ∂ρ/∂t is some value, a constant for simplicity, say Jsink. This gives you an equation for the flow of water. Now, just determine the coupling of the water to the boat (non-trivial), and you can determine the effect on the boat. Of course, the boat will probably introduce whacky boundary conditions when you solve the diff. eq. so I don't think this is an easy problem to solve in detail. I would imagine, as a first guess, that the boat wouldn't go anywhere.
 
  • #10
I agree with what Integral and Krab state about the non-physicality of the question, but don't we spend much of our time in Physics considering frictionless surfaces, point masses, infinite distances, field free areas etc...

This helps us to understand the Physics, but they are hardly 'genuine' circumstances are they?
 

What is the relationship between a boat in water and pressure?

When a boat is placed in water, it displaces a certain amount of water due to its weight. This displacement creates an upward force called buoyancy, which acts against the force of gravity. This buoyant force results in pressure being exerted on the boat, pushing it upwards and keeping it afloat.

How does pressure affect the motion of a boat in water?

The movement of a boat in water is affected by the pressure distribution around it. As the boat moves, it creates areas of high pressure on the front and low pressure on the back. This creates a force called drag, which acts in the opposite direction of the boat's motion. The shape and design of the boat are important in minimizing drag and improving its speed and efficiency.

Why does a boat float on water?

A boat floats on water because of the principle of buoyancy. The weight of the boat is equal to the weight of the water it displaces. This creates a balance between the downward force of gravity and the upward force of buoyancy, keeping the boat afloat. The shape and density of the boat also play a role in its ability to float.

How does the shape of a boat affect its motion in water?

The shape of a boat is crucial in determining its motion in water. A streamlined shape reduces drag and allows the boat to move more efficiently through the water. Additionally, the shape of the hull, or the bottom of the boat, can affect its stability and maneuverability. Different shapes and designs are used for different types of boats, depending on their intended use.

What other factors besides pressure and motion affect a boat in water?

Other factors that can affect a boat in water include wind, current, and waves. These external forces can impact the speed, direction, and stability of the boat. Additionally, the weight and distribution of weight on the boat can affect its buoyancy and performance in the water. Weather conditions and water temperature can also play a role in the boat's motion.

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