Naked Science Forum
Non Life Sciences => Technology => Topic started by: neilep on 30/11/2007 16:45:05
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Dear Jetologists,
How are ewe today ?...I'm Ok....my ear is still giving me grief !..thanks for asking though !!
Here's Brian just putting the finishing touches to one of my jet engines about to installed onto my remote control toy plane !
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But, I'm confused (no change there [;D]).......When I instruct Brian to test my engine he will have to stand in front of the engine which will cause him to be sucked into Jet-Engine joy........Once Brian has been sent to the repair shop he'll then have to stand behind the engine which will then (in addition to ruining his hair style) blow him a few hundred feet.
This might ruin his day ! (hmmmm...I think he's peering over my shoulder as I type this)
So, my question (Finally)...does a jet engine push an aeromobile or pull it ?
I would ask Brian but for some reason he's run off !!
hmmmmmmmm !!!
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I suppose that strictly speaking, all things (aerodynamically speaking) are pushed, and never pulled.
When poor Brian (where did you find such a hapless guy) gets sucked into the engine intake, what is really happening is that normal atmospheric pressure is actually pushing him into the jet intake. Normally, you would have pressure ahead of Brian, and pressure behind him, so as much as he gets pushed in one way, so he is pushed in the other. Then you spin the jet engine up, the impeller removes the air pressure on one side of Brian, so the pressure on the other side of Brian then pushes his head into the impeller, and you are left with minced Brian (or is that minced brain - I cannot tell).
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So, my question (Finally)...does a jet engine push an aeromobile or pull it ?
It depends on where you put it: if you put it in front of the aeromobile, this will be pulled, if you put it at the end, it'll be pushed, if you put it in the middle...both. [:)]
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Neither.
If you sit on a chair with wheels with a bag of cement and throw it your chair will move in the opposite direction. Newtons 3rd law. "To every action force there is an equal, but opposite, reaction"
To push you need something to push against and the air around us does not provide enough resistance for the thrust of the engine to push against and produce motion.
In other words a jet engine dosent move a plane because its pulling its self forward as it sucks air in and its not moving forward because its thrust is pushing against the air behind its because its simply throwing mass out of the back of the engine causing the plane to move in the opposite direction due to newtons 3rd law
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Think about this one ,how does a rocket in space move forward.
It cant be pushing as their is nothing to push against.
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Think about this one ,how does a rocket in space move forward.
It cant be pushing as their is nothing to push against.
Think about this one, a plane travelling at 1000mph launches a rocket that also flies at 1000mph...what happens to the rocket?
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Ok i though about it.
The rocket will eventually run out of fuel after fying at 1000 mph for a while. [;D]
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OK, so let's tackle the rocket in space first.
Newton's 3rd Law is that every action has an equal and opposite reaction, right? Now, when a rocket is firing it is throwing pressurized exhaust gas (meaning that there's a whole lot coming out at once) out the back end at extremely high speeds. You can find the instantaneous acceleration that gives your rocket by F=ma.
In other words, the rocket is pushed by the exhaust.
Rocket launched from an aircraft: Make sure that you say relative to whom. Let's take this discussion to space to make it easier.
The plane traveling 1000MPH (relative to an external observer) sees the rocket traveling at 0MPH on its wingtip. Once the rocket is launched and begins to accelerate to 1000MPH relative to the aircraft (it has a governor to stop it once it gets to 1000MPH). When it gets to 1000MPH relative to the plane, it is traveling at 2000MPH relative to the external observer.
In essence, you're correct, but be careful.
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Neither.
If you sit on a chair with wheels with a bag of cement and throw it your chair will move in the opposite direction. Newtons 3rd law. "To every action force there is an equal, but opposite, reaction"
To push you need something to push against and the air around us does not provide enough resistance for the thrust of the engine to push against and produce motion.
In other words a jet engine dosent move a plane because its pulling its self forward as it sucks air in and its not moving forward because its thrust is pushing against the air behind its because its simply throwing mass out of the back of the engine causing the plane to move in the opposite direction due to newtons 3rd law
Yes, but the question was "does a jet engine push an aeromobile or pull it ?" So it doesn't matter how the engine works, if you put it in front of the aeromobile the engine will make a forward force in that point, so it will pull it and the aeromobile's materials will be stretched; if you put the engine in the back, the aeromobile's materials will be compressed.
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Yes, I agree. My post was more towards clearing up the misconception that rockets cannot accelerate in space and clarifying inertial reference frames.
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There's no such force as 'suck' if you want to be purist. You can only have excess pressure on one side, which will push things. Gases can't pull because the molecules are miles apart. There are lots of smartarse demonstrations that show you that you need pressure 'on the other side' .
e.g. putting a sealed lid over a cup and you can't suck the drink out.
It's got to be true that you can only have 1 atmosphere of excess pressure at the front of the engine but loads of excess pressure at the back.
Reaction engines can be conveniently explained in terms of momentum. Total momentum is always conserved. If you eject a small mass of hot exhaust gases at high velocity, backwards, your rocket (high mass) will end up going forwards with a smaller velocity.
Mass times velocity will be the same in both directions - giving a net momentum change of zero.
Don't ya just love School Science?