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09/16/2011 10:38:22 AM · #26 |
Originally posted by mike_311:
OK now that that is solved, how about a new one:
Imagine you are sitting in a boat swimming in a pool. You have got a big rock in your hand. When you throw the rock into the pool what happens to the water level of the pool? |
Nothing, aside from some small waves. The displacement is equal, now if you throw it in from the shore there is an increased displacement.
Next? |
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09/16/2011 10:40:51 AM · #27 |
This.
And, while it would depend on the type of plane, the majority wouldn't take off as there's no lift (despite the engine providing forward momentum). The wings are essentially useless here (given a fair experiment where the conveyor is synchronised with the planes forward momentum - unlike the mythbusters experiment). The plane would need to outrun the conveyor by at least a small amount for air to pass under the wings and provide enough lift for the plane to leave the ground and nullify the conveyor.
I look forward to someone proving me wrong. |
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09/16/2011 10:41:51 AM · #28 |
Originally posted by Cory:
Nothing, aside from some small waves. The displacement is equal, now if you throw it in from the shore there is an increased displacement.
Next? |
wrong. any other takers? |
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09/16/2011 10:45:23 AM · #29 |
| Surely it depends on the mass of the rock relative to its volume? The pool's water level could increase, decrease or stay the same depending on this. |
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09/16/2011 10:46:35 AM · #30 |
In fact due to surface tension any objects too light would float... so assuming the rock doesn't float (can happen), then the pool level will decrease. I guess that's right, let me spend more than 10 seconds thinking about it now =).
Message edited by author 2011-09-16 10:48:09. |
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09/16/2011 10:49:58 AM · #31 |
Originally posted by mike_311:
Originally posted by Cory:
Nothing, aside from some small waves. The displacement is equal, now if you throw it in from the shore there is an increased displacement.
Next? |
wrong. any other takers? |
Well, I suppose if I read your post literally, the pool is in the boat, not the boat is in the pool.... but I had assumed that you just worded this poorly and weren't simply trying to play word games. |
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09/16/2011 10:52:04 AM · #32 |
Originally posted by HawkinsT:
The plane would need to outrun the conveyor by at least a small amount for air to pass under the wings and provide enough lift for the plane to leave the ground and nullify the conveyor.
I look forward to someone proving me wrong. |
Okay. Imagine the plane is on the conveyor belt, wheels spinning like mad, but sitting in the same place.
So you go over and attach a strong rope to the front of the plane, stand in front of the conveyor belt, and pull really hard - What happens?
Now, another way of looking at it. Imagine you're on a pair of skates on an ice-rink, and the ice-rink is moving backwards to match your forward motion. Same idea, you grab a rope and start pulling yourself forwards. No matter how fast the ice-rink moves backwards it's not going to stop you.
The props/engines are doing exactly the same thing as the rope. |
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09/16/2011 10:52:14 AM · #33 |
Originally posted by HawkinsT:
Surely it depends on the mass of the rock relative to its volume? The pool's water level could increase, decrease or stay the same depending on this. |
Lets assume it's more dense, since it almost always is.
the rock displaced is weight in water volume in the boat while it only displaces its own volume in the water, since the rock is more dense than water, the water volume displaced is greater so the water level will decrease. |
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09/16/2011 10:54:33 AM · #34 |
Originally posted by JH:
Now, another way of looking at it. Imagine you're on a pair of skates on an ice-rink, and the ice-rink is moving backwards to match your forward motion. Same idea, you grab a rope and start pulling yourself forwards. No matter how fast the ice-rink moves backwards it's not going to stop you.
The props/engines are doing exactly the same thing as the rope. |
i was going to say just picture a plane that takes off and lands on the water, they do right? nearly same scenario. the plane inst propelled against the water, it just glides on top of it. |
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09/16/2011 10:54:47 AM · #35 |
Originally posted by mike_311:
Originally posted by HawkinsT:
Surely it depends on the mass of the rock relative to its volume? The pool's water level could increase, decrease or stay the same depending on this. |
Lets assume it's more dense, since it almost always is.
the rock displaced is weight in water volume in the boat while it only displaces its own volume in the water, since the rock is more dense than water, the water volume displaced is greater so the water level will decrease. |
Lol, you are actually correct.. that's rather funny, and I should have though of the SG issue. |
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09/16/2011 10:56:22 AM · #36 |
Originally posted by Cory:
Originally posted by mike_311:
Originally posted by Cory:
Nothing, aside from some small waves. The displacement is equal, now if you throw it in from the shore there is an increased displacement.
Next? |
wrong. any other takers? |
Well, I suppose if I read your post literally, the pool is in the boat, not the boat is in the pool.... but I had assumed that you just worded this poorly and weren't simply trying to play word games. |
lol, i see that now... it wasn't intentionally worded to to be tricky. there is actual science behind it. |
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09/16/2011 11:00:19 AM · #37 |
Originally posted by JH:
Originally posted by HawkinsT:
The plane would need to outrun the conveyor by at least a small amount for air to pass under the wings and provide enough lift for the plane to leave the ground and nullify the conveyor.
I look forward to someone proving me wrong. |
Okay. Imagine the plane is on the conveyor belt, wheels spinning like mad, but sitting in the same place.
So you go over and attach a strong rope to the front of the plane, stand in front of the conveyor belt, and pull really hard - What happens?
Now, another way of looking at it. Imagine you're on a pair of skates on an ice-rink, and the ice-rink is moving backwards to match your forward motion. Same idea, you grab a rope and start pulling yourself forwards. No matter how fast the ice-rink moves backwards it's not going to stop you.
The props/engines are doing exactly the same thing as the rope. |
But assuming the engine is angled parallel to the ground and so is generating no lift itself (I realise in many planes they are angled slightly upwards), then as long as the conveyor belt is able to match the thrust generated, it will also be providing equal resistance to the planes forward momentum, so the plane will stay static, no? The plane will only take off when it can actually outrun the conveyor belt (as happened in that mythbusters clip) - which the questions states doesn't happen. |
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09/16/2011 11:01:41 AM · #38 |
| Pictures! Pictures! Where or where are the pictures?! |
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09/16/2011 11:02:51 AM · #39 |
Originally posted by HawkinsT:
But assuming the engine is angled parallel to the ground and so is generating no lift itself (I realise in many planes they are angled slightly upwards), then as long as the conveyor belt is able to match the thrust generated, it will also be providing equal resistance to the planes forward momentum, so the plane will stay static, no? The plane will only take off when it can actually outrun the conveyor belt (as happened in that mythbusters clip) - which the questions states doesn't happen. |
you are assuming that the wheels are actually doing something, the wheels are there so the plane doesn't scape along the ground while the turbines pull it through the air.
Message edited by author 2011-09-16 11:03:18. |
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09/16/2011 11:06:16 AM · #40 |
Actually the only situation it changes is when the rock is denser than water, it rises. In the other situations the rock floats and displaces its mass as it does in the boat, so no change. When denser, it sinks it displaces its volume not its weight (which because its denser is less displacement than its mass).
Originally posted by HawkinsT:
Surely it depends on the mass of the rock relative to its volume? The pool's water level could increase, decrease or stay the same depending on this. |
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09/16/2011 11:10:14 AM · #41 |
Originally posted by mike_311:
Originally posted by HawkinsT:
But assuming the engine is angled parallel to the ground and so is generating no lift itself (I realise in many planes they are angled slightly upwards), then as long as the conveyor belt is able to match the thrust generated, it will also be providing equal resistance to the planes forward momentum, so the plane will stay static, no? The plane will only take off when it can actually outrun the conveyor belt (as happened in that mythbusters clip) - which the questions states doesn't happen. |
you are assuming that the wheels are actually doing something, the wheels are there so the plane doesn't scape along the ground while the turbines pull it through the air. |
The wheels are doing something, they are in contact with the backwards-moving conveyor and so creating resistance... you can prove this by shutting off the planes engines while keeping the conveyor going - the plane will move backwards. |
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09/16/2011 11:11:00 AM · #42 |
Originally posted by millsa:
Actually the only situation it changes is when the rock is denser than water, it rises. In the other situations the rock floats and displaces its mass as it does in the boat, so no change. When denser, it sinks it displaces its volume not its weight (which because its denser is less displacement than its mass).
Originally posted by HawkinsT:
Surely it depends on the mass of the rock relative to its volume? The pool's water level could increase, decrease or stay the same depending on this. |
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Yeah, hence my second post right after my first, I saw my mistake =). |
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09/16/2011 11:14:58 AM · #43 |
Originally posted by HawkinsT:
The wheels are doing something, they are in contact with the backwards-moving conveyor and so creating resistance... you can prove this by shutting off the planes engines while keeping the conveyor going - the plane will move backwards. |
only becuase of friction caused by the weight of the plane. unless that friction force exceeds the inertia of the plane the conveyor and wheels will spin and the plane wont move backward.
Message edited by author 2011-09-16 11:15:27. |
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09/16/2011 11:19:49 AM · #44 |
Originally posted by HawkinsT:
Originally posted by mike_311:
Originally posted by HawkinsT:
But assuming the engine is angled parallel to the ground and so is generating no lift itself (I realise in many planes they are angled slightly upwards), then as long as the conveyor belt is able to match the thrust generated, it will also be providing equal resistance to the planes forward momentum, so the plane will stay static, no? The plane will only take off when it can actually outrun the conveyor belt (as happened in that mythbusters clip) - which the questions states doesn't happen. |
you are assuming that the wheels are actually doing something, the wheels are there so the plane doesn't scape along the ground while the turbines pull it through the air. |
The wheels are doing something, they are in contact with the backwards-moving conveyor and so creating resistance... you can prove this by shutting off the planes engines while keeping the conveyor going - the plane will move backwards. |
No that's friction doing something, without it the wheels would continue to spin happily at the speed they were at. part of the engines thrust is overcoming that friction.
As I mentioned it is physically and mathematically impossible to set up the experiment where the conveyor matches the speed of the wheels. Turn on a belt sander then push a matchbox car back and forth on it. Regardless of how fast the belt (ignoring the friction that may rip it from your hand and embed it in the wall) you can freely move the car back and forth with the same force. Your hand is the same as the thrust from the engine.
Message edited by author 2011-09-16 11:22:38. |
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09/16/2011 11:22:19 AM · #45 |
| So essentially what we have here is you assume the plane will take off based on it being able to move forward, which could only come about by the wheels slipping. I on the other hand, am asserting the plane will not take off due to it not being able to move forward as I assume the wheels don't slip... so we are simply interpreting the poorly-defined system differently. Would you agree on this? |
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09/16/2011 11:26:34 AM · #46 |
Originally posted by HawkinsT:
So essentially what we have here is you assume the plane will take off based on it being able to move forward, which could only come about by the wheels slipping. I on the other hand, am asserting the plane will not take off due to it not being able to move forward as I assume the wheels don't slip... so we are simply interpreting the poorly-defined system differently. Would you agree on this? |
The wheels don't slip, the question is impossible to produce in the real world, the conveyor can never match the speed of the wheels once a thrust is applied. This is because the speed of the wheels is always the speed of the conveyor + the speed from the forward velocity of plane. ie always greater than the conveyor. |
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09/16/2011 11:29:09 AM · #47 |
| Yes, but it's a hypothetical question. So you interpret it as the conveyor originally matching the planes forward momentum but then the plane outruns it, I interpret the question as the conveyor can not be outrun by the plane. |
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09/16/2011 11:31:54 AM · #48 |
the conveyor belt wont be able to generate enough force on the plane to keep it in place compared to the force the thrust will generate, since the thrust is tied directly to the plane and the force from the conveyor needs to be transmitted through the wheels.. the wheels will spin faster than usual but the plane will still move forward and eventually take off...
the belt sander is a great analogy, btw. |
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09/16/2011 11:32:29 AM · #49 |
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09/16/2011 11:36:11 AM · #50 |
Originally posted by HawkinsT:
Yes, but it's a hypothetical question. So you interpret it as the conveyor originally matching the planes forward momentum but then the plane outruns it, I interpret the question as the conveyor can not be outrun by the plane. |
True, but in the hypothetical universe where the conveyor can not be outrun by the plane, the laws of physics of this universe obviously don't apply! So why not choose to assume that the mystical powers of the pilot make the aircraft fly and therefore it still takes off. |
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