Easy physics problem

ark13
Posers and Puzzles 07 Nov '07 22:23
1. ark13
Enola Straight
07 Nov '07 22:231 edit
Let's say you have a car (or an airplane) powered by a propeller. Your original design featured a large wall, attached to the car (or airplane), behind the propeller, which is hit by the air coming from the back of the propeller. Your friend says you should remove the wall, because it'll make the car (or airplane) go faster. You say it doesn't make a difference. Who's right? Explain your reasoning.
2. Lukerik
07 Nov '07 22:39
Before we go and research this, how far behind the propeller is the wall?
3. ark13
Enola Straight
07 Nov '07 22:58
Originally posted by Lukerik
Before we go and research this, how far behind the propeller is the wall?
Right behind it, or anywhere; I don't think it should matter. I'm not talking about numbers, I'm talking about principles. If any extra air hits the wall, the principle applies.
4. coquette
08 Nov '07 01:40
yes, faster ..but only in the impossible theoretical sense. "the wall" is "pushing back" on the molecules, each in turn "pushing back" .. .really meaning pushing forward, moving the "flying, sailing, gliding, rolling" object forward.
5. 08 Nov '07 04:00
The air molecules are being pushed backward by the prop which pushes the plane forward. These fast moving molecules then hit the wall pushing the wall backwards, which of course pushes the plane backwards. So yeah, removing the wall will speed it up. Seems too simple... am I missing something???
6. AThousandYoung
08 Nov '07 06:47
Originally posted by XZantoth
The air molecules are being pushed backward by the prop which pushes the plane forward. These fast moving molecules then hit the wall pushing the wall backwards, which of course pushes the plane backwards. So yeah, removing the wall will speed it up. Seems too simple... am I missing something???
no, u got it
7. sven1000
Astrophysicist
08 Nov '07 07:13
Not to mention that removing the wall removes a lot of air resistance drag on your car/plane, regardless of your propulsion system! It would be like driving/flying with an anchor.
8. smw6869
Granny
08 Nov '07 16:54
Wrong-o-Buckos! Propellers don't push, they suck. The thrust of the engines move the plane forward. The propellers act as a truck you are drafting behind. This is all simple physics.

Granny.
9. uzless
The So Fist
08 Nov '07 19:53
Originally posted by ark13
Let's say you have a car (or an airplane) powered by a propeller. Your original design featured a large wall, attached to the car (or airplane), behind the propeller, which is hit by the air coming from the back of the propeller. Your friend says you should remove the wall, because it'll make the car (or airplane) go faster. You say it doesn't make a difference. Who's right? Explain your reasoning.
simple. The wall has mass. By removing the wall you are removing mass so your speed will increase.

Anytime you reduce weight you will increase speed.
10. ark13
Enola Straight
08 Nov '07 21:56
Originally posted by XZantoth
The air molecules are being pushed backward by the prop which pushes the plane forward. These fast moving molecules then hit the wall pushing the wall backwards, which of course pushes the plane backwards. So yeah, removing the wall will speed it up. Seems too simple... am I missing something???

It was supposed to be easy; maybe even deceptively easy. People tend to over-think this sort of thing.
11. 08 Nov '07 23:54
Originally posted by ark13
Let's say you have a car (or an airplane) powered by a propeller. Your original design featured a large wall, attached to the car (or airplane), behind the propeller, which is hit by the air coming from the back of the propeller. Your friend says you should remove the wall, because it'll make the car (or airplane) go faster. You say it doesn't make a difference. Who's right? Explain your reasoning.
What is the wall made out of?

IRNG 24
12. sonhouse
Fast and Curious
10 Nov '07 23:58
Originally posted by ark13
Let's say you have a car (or an airplane) powered by a propeller. Your original design featured a large wall, attached to the car (or airplane), behind the propeller, which is hit by the air coming from the back of the propeller. Your friend says you should remove the wall, because it'll make the car (or airplane) go faster. You say it doesn't make a difference. Who's right? Explain your reasoning.
I don't see any thrust being developed at all, seems to me the wall would deflect the air trying to leave the vicinity of the prop which would produce thrust if all the air goes in the same direction. But when a wall is there, the air is all forced sideways, downwards, upwards and diverting the thrust to 90 degrees from the original direction of unimpeded air flow. Now if the wall had a piece sticking out from it in such a way as to block the flow of air in certain directions, you could get a twisting force or possibly a lifting force if the air was directed straight down but with just a wall it seems to me the thrust would be vectored off in all directions, think of a stream of water from a hose hitting a wall, its going to splatter all sideways and will try to move the wall a bit but that would be in the opposite direction. It would be like trying to put a giant electric fan on a sailboat, no net thrust. If anything it looks to me like it might go backwards.
13. 11 Nov '07 22:20
Originally posted by Doctor Rat
What is the wall made out of?
Because, what if the wall is made out of, like, you know, more propellors?
I do all my best thinking on the bus.
14. 11 Nov '07 23:52
Originally posted by ark13
Let's say you have a car (or an airplane) powered by a propeller. Your original design featured a large wall, attached to the car (or airplane), behind the propeller, which is hit by the air coming from the back of the propeller. Your friend says you should remove the wall, because it'll make the car (or airplane) go faster. You say it doesn't make a difference. Who's right? Explain your reasoning.
how can the pilot see with a huge wall infront of him? assuming that the prop is in front of the driver, and the wall is behind the prop, we have bigger problems than how fast it will go.
15. 12 Nov '07 02:14
Originally posted by rubberjaw30
how can the pilot see with a huge wall infront of him? assuming that the prop is in front of the driver, and the wall is behind the prop, we have bigger problems than how fast it will go.
lol

What is the driver made out of?