A plane on a runway, how smart is r3vlimited?

[Erick]

God my head hurts with all this thinking at 2am.... finals were last week, I dunno why I end up in the same scenario again. -.-

[Justin B]

Yeah, I can see how people would think it would either stay the same (how?? lol), or go up because now the bowling ball is now in the water too, but not taking into account for the boat rising out of the water from the relieved weight. I'd say that one was a little easier to see in my head.

[george graves]

You got it.

[Justin B]

good but easy...

Ok here is a better one: A guy is sitting in a row boat, that is floating in a pool full of water. He is holding a bowling ball. He throws the bowling ball overboard and it(the bowling ball) sinks to the bottem. Now, what had happened to the level of the water in the pool?

It went down right? When the bowling ball was in the boat the water was being displaced by its weight, while when it was out of the boat and in the water, the boat rose up from the relieved weight and the water was displaced by the bowling ball that was just dropped in, but the bowling ball displaced less water by its volume than by its weight in water. The difference in the water level would depend on the weight of the ball.


Edit - Ok, good I'm right. I started this post before the most recent posts above :)

[george graves]

VERY GOOD! Yes, the ball has to displace MORE water when it is in the boat cause it weighs MORE than water. So it must displace a VOLUME of water equale to it's weight. When it's on the bottem of the pool - it's resting on the bottem - and it only displaces it's VOLUME (NOT the the VOLUME of water equale to it's weight)

[joshh]

nope.

Well then I've been thinking for beyond what I should be in one night.

The ball displaces more water in the boat than it does one for one with the weight of the water??

[george graves]

That one is way easier. The boat has to displace the same amount of water weight as the weight of the boat. Nothing happens at all.

nope - get your physics book out











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DON"T READ AHEAD IF YOU WANT DON"T WANT THE ROW BOAT ANSWER

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[george graves]

Hahaha, it momentarily dropped while the ball was in the air, and then it returned to the previous level :)

Nope! (PS - don't worry about while it's in the air - just before and after)

[Justin B]

Sorry, but you are wrong. The conveyor belt is not keeping the plane stationary.
The wheels on a plane are simply there to roll. No power is going though the wheels. All the conveyor belt is doing is spinning the wheels really fast. It has no effect on the speed of the plane.
-Erik

Oh my god. That light finally came on in my head and I FINALLY figured out what the hell all this was about free spinning wheels. DUH.

The plane itself will move forward. Then lift off.

Anybody thinking about that the plane would stay stationary (and obviously wouldn't lift at all if that was the case unless the prop is making a shit ton of lift over the WINGS I think), here's another way to say it. Instead of the plane and the conveyor belt gaining speed at the same time (what you would automatically think), imagine this. Plane is stationary, conveyor belt speeds up really fast to whatever speed it needs to be at. We'll assume that the planes wheels are ultra low drag and somehow it doesn't start moving backwards. Wheels are spinning in the opposite direction as the belt now, at the same speed. Keep in mind the plane is still stationary. Once the plane starts up and creeps forward, these crazy wheels are spinning at the ground speed PLUS the regular ground speed that it should be experiencing from the forward thrust from the prop/jet engine. The plane will be moving forward along what would have to be a terribly long conveyor belt runway. All it turns out to be is undue stress on the wheels, everything else is ordinary.

So, so simple....

[Axxe]

I can see it both ways but frankly I don't know which is right.

My way :pimp:

[RobertK]

The wheels have no real effect on the plane other than holding it up and providing a theoretical frictionless surface. Now of course it isn't frictionless, however when you are trying to max speed a car (say an e30), do you hit max speed because of bearing drag or because of aerodynamic drag? The force of the air at speed increases at a rate squared to the speed, so it is clearly the most resistance the plane feels.

That's like comparing apples and oranges.

I guess we need to agree first if this is based on a theoretical setup or a realistic one. Right now the friction created by the wheels is the setback. If they are frictionless then the answer is easy.

[joshh]

Read Axxe's confirmation ^^^

If the engines are off, then the plane is stopped and the belt wont spin.... the belt works against any foward motion that the plane is producing... if the plane isnt producing (engines off) then the belt also wont be moving.

I can see it both ways but frankly I don't know which is right.

[808BMW]

The thrust of the engines have to make the wheels go forward don't they. The weight of the plane sits on the wheels correct? ......so if the wheels are already spinning as fast as the engines can propel the aircraft forward, the plane will not take off.
The treadmill simulates the power of the engines forcing the plane mass forward.

The thrust of the engines make the plane go forward.
A forward moving plane and traction of the tire on the stationary ground/rolling conveyor make the wheels "roll forward".

It's pretty much a trick question

[Axxe]

No.. the wheels are like gears plus since nothing is 100% efficient at converting energy drag would be created in the xfer of energy from the forward force of the plane and the backward force of the treadmill. In reality the bearing would eventually burn up or the tires would blow out.

The wheels have no real effect on the plane other than holding it up and providing a theoretical frictionless surface. Now of course it isn't frictionless, however when you are trying to max speed a car (say an e30), do you hit max speed because of bearing drag or because of aerodynamic drag? The force of the air at speed increases at a rate squared to the speed, so it is clearly the most resistance the plane feels.

[joshh]

good but easy...

Ok here is a better one: A guy is sitting in a row boat, that is floating in a pool full of water. He is holding a bowling ball. He throws the bowling ball overboard and it(the bowling ball) sinks to the bottem. Now, what had happened to the level of the water in the pool?

That one is way easier. The boat has to displace the same amount of water weight as the weight of the boat. Nothing happens at all.