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Geek Culture / puzzle: two retorts filled with water...

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Dark Java Dude 64
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Posted: 5th Dec 2013 01:50
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Yes exactly. If it was propelling itself by its wheels and only its wheels however, no external thrust source, it wouldn't be able to move forward.

But as we know, planes are not like that, so yes, any plane would be able to take off on a conveyor built.

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mr Handy
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Posted: 5th Dec 2013 07:03
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Quote: "Has nobody mentioned that myth busters already did this?"

Their tests are not 100% correct, some just was pulling facts to certain results.

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BatVink
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Posted: 6th Dec 2013 14:00
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Quote: "Has nobody mentioned that myth busters already did this?"


That wasn't the experiment as described here. they matched the plane's speed to the belt. In this problem they ask you to match the wheel speed.

In the Mythbusters experiment the wheels were going Belt Speed + Plane Speed m/s.

As mentioned previously the belt can never match the wheel speed as soon as the plane is > 0 m/s.

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Indicium
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Posted: 6th Dec 2013 18:00
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Quote: "they matched the plane's speed to the belt. In this problem they ask you to match the wheel speed."


Oh - everything makes sense now.


They see me coding, they hating. http://indi-indicium.blogspot.co.uk/
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Dark Java Dude 64
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Posted: 6th Dec 2013 21:43
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So the other day I was sitting on my couch, and marveling at the interesting and inspiring physics questions that have been asked here.

So I thought up my own physics question, and I really can't seem to figure it out. It goes like this: Suppose I were standing on a sail boat, and I started blowing on the sail very hard. Or better yet, I take a large fan, put it on the boat, aim it at the sail, and turn it on. Will the boat move?

Another one: say we had a special type of airplane. This plane would have the wings not outside of the plane, but inside the plane, in a special lift chamber. Now we take a fan mounted inside the plane, and aim it at the internal wings. Will it take off?

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mr Handy
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Posted: 6th Dec 2013 21:49
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Quote: "Suppose I were standing on a sail boat, and I started blowing on the sail very hard.
Will the boat move?"

Yes but no, only Chuck Norris can do that.
Quote: "Or better yet, I take a large fan, put it on the boat, aim it at the sail, and turn it on.
Will the boat move?"

Yes.

Quote: "This plane would have the wings not outside of the plane, but inside the plane, in a special lift chamber. Now we take a fan mounted inside the plane, and aim it at the internal wings. Will it take off?"

No, closed system. You also could try the classics - grab your hair and lift yourself up.

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BatVink
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Posted: 6th Dec 2013 22:00
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Quote: " Suppose I were standing on a sail boat, and I started blowing on the sail very hard. Or better yet, I take a large fan, put it on the boat, aim it at the sail, and turn it on. Will the boat move?"


A large fan, sounds rather like a propeller?


Quote: "This plane would have the wings not outside of the plane, but inside the plane, in a special lift chamber"


When you test an aircraft in a wind tunnel, does the wind tunnel take off?

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Dark Java Dude 64
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Posted: 6th Dec 2013 22:02
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Ah, my mental toils are over! Thanks a bunch friends.

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mr Handy
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Posted: 6th Dec 2013 22:55
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Solve this: in Africa, in the hot parts of the continent, overhead power lines are made very thick, ie much larger cross-section than in the rest of the land. Why?

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easter bunny
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Posted: 6th Dec 2013 23:06
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Quote: "Suppose I were standing on a sail boat, and I started blowing on the sail very hard. Or better yet, I take a large fan, put it on the boat, aim it at the sail, and turn it on. Will the boat move?"

No.

Quote: "in Africa, in the hot parts of the continent, overhead power lines are made very thick, ie much larger cross-section than in the rest of the land. Why?"

Expansion?


Audacia Games
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mr Handy
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Posted: 6th Dec 2013 23:22
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Quote: "No."

Orly? reaction propulsion, heard of?

Quote: "Expansion?"

What?

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easter bunny
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Posted: 6th Dec 2013 23:40 Edited at: 6th Dec 2013 23:45
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Quote: "Orly? reaction propulsion, heard of?"

Nope.
But think about it like this:
How does an aeroplane move?
It blows air out the back with a fan!
This is Newtons law "every action has an equal and opposite reaction". The air moving backwards moves the plane forwards.
So the fan on the boat blowing towards the sail will blow the boat in the opposite direction to the sail.
BUT, the air blowing against the sail will push the boat in the SAME DIRECTION as the sail!
These 2 opposite forces will cancel each other out. Meaning to boat will go nowhere.

Quote: "Orly? reaction propulsion, heard of?"

Edit: Just didn't know it by name...

edit2: This is simply the theoretical explantion of it.
In reality, it's sometimes different

Quote: "What?"

Does it have something to do with the expansion of the wires? (They will expand when they get hotter, and shrink when they get cooler). That's why there are the gaps in railway lines, and the bumps at each end of a metal bridge, to compensate for expansion.


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Seditious
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Posted: 6th Dec 2013 23:49
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Quote: "BUT, the air blowing against the sail will push the boat in the SAME DIRECTION as the sail!"


It doesn't matter, air is still being pushed around and has to go somewhere, so the boat will move in one direction or another.
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easter bunny
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Posted: 7th Dec 2013 00:42 Edited at: 7th Dec 2013 00:45
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Quote: "It doesn't matter, air is still being pushed around and has to go somewhere, so the boat will move in one direction or another. "


Quote: "edit2: This is simply the theoretical explantion of it.
In reality, it's sometimes different"


It will theoretically end up coming out the sides in equal proportions (forces again counteracting each other), this will depend on the exact position of the fan and such though


Audacia Games
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Dark Java Dude 64
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Posted: 7th Dec 2013 01:16 Edited at: 7th Dec 2013 01:22
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I'm not sure if you guys realized this or not, but I was trolling when I posted that post with those two questions. It's sort of sad that my troll intentioned statements are being debated over.

As for the boat, it shouldn't move. The fan will accelerate the air to a specific velocity, and because the air has mass, the fan will need to apply a force to the air to do so. The air will apply an equal and opposite force to the fan, pushing it backwards. At the same time, when the air hits the sail, its velocity will be removed by the sail. The sail will have to apply a force to the air to do so, and again, the air will apply an equal and opposite force to said sail, pushing it forwards. With how much force? Same amount as the fan is being pushed backwards with. Boom, forces cancel out, zero net force, no acceleration of the boat, no movement.

I am of course assuming ALL of the air blown by the fan is stopped by the sail. If any portion of the air blown by the fan gets to go around the sail, then the boat will be propelled in the opposite direction the fan is facing, and would do even more so if the sail was not present. As BatVink said, the fan is like a propeller.

So situation depending, the boat may or may not move. Really need to take into account all variables.

Quote: "It doesn't matter, air is still being pushed around and has to go somewhere, so the boat will move in one direction or another."
If the sail directs air straight up and straight to the sides only (it probably won't though), then the boat shouldn't move.

Quote: "in Africa, in the hot parts of the continent, overhead power lines are made very thick, ie much larger cross-section than in the rest of the land. Why?"
My guess would be that as conductors get warmer, they tend to not conduct as well. A thicker wire for a given conductivity level makes for a more conductive wire. The more conductive the wire, the more efficiently power is transferred.

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Seditious
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Posted: 7th Dec 2013 01:25
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Quote: "If the sail directs air straight up and straight to the sides only (it probably won't though), then the boat shouldn't move."


A completely flat sail wouldn't be very useful. I'd imagine that due to the bow shape the air would be reflected at an angle. It'd probably be much more efficient to just point the fan in the opposite direction, though.
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Dark Java Dude 64
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Posted: 7th Dec 2013 01:28
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Lol, yes indeed.

I think a bowed sail is generally better as it stops the air more effectively.

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Kezzla
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Posted: 7th Dec 2013 05:06
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Quote: "in Africa, in the hot parts of the continent, overhead power lines are made very thick, ie much larger cross-section than in the rest of the land. Why?"


Does it have anything to do with monkeys?

Burn retina, burn!
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BatVink
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Posted: 7th Dec 2013 13:04
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Quote: "It's sort of sad that my troll intentioned statements are being debated over."


I think it's quite interesting and good, it challenges the assumptions we make everyday. As a species we are inherently irrational (fact not opinion!)

The propeller theory is a good one to stick to. It does 2 things:

1. It creates low pressure behind it (behind being in front of the plane or behind the boat). The low pressure must be equalised, so it draws things towards it. For the plane this is good news, it pulls it forwards. For the boat this is bad news, it would make it go backwards. So why does the boat still move forwards?

2. The boat moves forwards because of the huge obstacle in front of the fan - the sail. Behind the fan, the low pressure can be filled from all directions, so the force on the boat to fill it is minimal. But in front of the fan, the sail is taking the full force of the pressure. Thus there is a minor backward force and a massive forward force.


This still leaves the question of how a plane can move forwards just on simple air pressure differences. There is another element to it. Propellers are shaped to push the air to a compressed area in the centre. At this point the propeller blades are pushing against a more condensed air mass, and it becomes more like pushing against a solid.

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Green Gandalf
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Posted: 7th Dec 2013 18:58 Edited at: 7th Dec 2013 19:00
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Real physics is sometimes counter-intuitive.

Try the following simple experiment and surprise yourself.

Obtain a drinking straw, a cotton reel, a drawing pin and a small square of card. Push one end of the straw through the centre of the cotton reel. Then push the drawing pin through the centre of the card. Then place the card against the cotton reel with the end of the pin inside the reel (see diagram).



Then loosely hold the card in place with your free hand and blow down the tube in the direction shown. While still blowing down the straw release the card and try to remove it by blowing hard. Then explain what happens.

As far as the fan/sail problem is concerned, I can only suggest that someone tries it.



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BatVink
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Posted: 7th Dec 2013 19:57
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That's a nice looking experiment. I'm guessing the effect is the same as blowing across the top of a piece of paper; it rather than falls because of the reduction in pressure.

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mr Handy
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Posted: 7th Dec 2013 20:24
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To all: Did you know that the pressure from the falling light is two times higher on the mirror than on light absorbing surface?

_________

About wires:
Quote: "Does it have anything to do with monkeys?"

Kezzla, you won!

_________

About boat:
In common case (powerful fan, regular sail) it will move. Physics aren't something you can solve just with your "opinion".



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Libervurto
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Posted: 8th Dec 2013 16:00
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Quote: "Suppose I were standing on a sail boat, and I started blowing on the sail very hard. Or better yet, I take a large fan, put it on the boat, aim it at the sail, and turn it on. Will the boat move?"

Yes and no, the fan is pulling the boat forward by pushing air out of it but all the air is then slamming straight into the sail and pushing the boat back in the opposite direction. Remove the sail and you have a fan boat moving one way, remove the fan and you have a sail boat moving the other.


Formerly OBese87.
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mr Handy
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Posted: 7th Jan 2014 10:28
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Small update (gif):



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