that explanation is way over my head.
but I should of thought that each would be a different price
any more brain teasers?

Nice easy one for Friday.

use spoiler command to hide the answer if you know it.


Jack has 2 friends. One that always lies and the other which never lies. One of them took his candy bar. What question can ask to either of the friends which will have the same outcome and he will know who took his candy bar.

Have fun.

Joe

Is it easier on Monday?

Nope. Still stuck.

the question to ask to either of them is: who would the other guy say took my candy bar. Whichever person is the answer.........it is the opposite.

"A plane is sitting on runway that can move (some sort of giant conveyer belt). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off? "

no
air speed over the foil (wing) determines lift, not ground speed

that is exactly why it will take off.

Q, I don't understand your answer.

From the perspective of someone standing beside the runway, they would see the wheels spinning but the plane would not be moving and thus there would be no air flowing over the wings.

I will give a clue.

The wheels are independant of the plane. They can spin freely.

Hi Q,

I just did a quick look at a science forum I used to visit, and guess what popped up?? Your question has generated lots of debate all over the net even on the physics forums – just Google it. However it seems a simple answer is not forthcoming. Everyone seems to have a different view and explanation.

My thought is that this has more due to with whether the paradox’s author meant to assume friction (of the wheel bearings & tires on the runway) was irrelevant. I can only guess so, but the educated explanations I’ve read almost always talk about it.

My simple explanation of why I posted my earlier answer of “no”…..

Firstly I made two assumptions which I believe were reasonable:
• We are using a normal airplane that you would find at any airport on this planet – that is, it is designed for normal flight as we expect it to occur.
• For the purposes of this question “take-off” is equivalent to “lift-off”, which I think is what would actually occur at first since there would be no forward progress relative the ground (notice I didn’t mention the conveyer/runway)
There are 4 forces applied to a plane during normal flight; gravity, lift, drag, and thrust. To achieve flight airflow over the wing creates lift. A plane’s engines can create airflow over the wing, but is there enough airflow even with the engines on maximum thrust to create the lift required to get the plane off the ground? No. A plane sitting on a normal runway with the wheels locked, and revs it’s engines up to full thrust does not lift off the ground. This is a common pre-flight test I believe. This is because the planes wings are designed to utilize the entire surface area. This would not actually occur when providing airflow from simply running-up the engines. Airplane wings are designed to provide lift across the entire wing. This is why tail-wind take-offs are discouraged!!
However, if the plane was re-engineered for the purposes of this paradox, possibly flight would be possible. Three possible design modifications that could be considered:
1) Engines that delivered the thrust airflow across the entire surface area of the wing,
2) Wing redesign so that a narrower wingspan could be utilized, at the point of thrust airflow,
3) Engine that can give unlimited thrust without increase in weight.

Cheers,
99gecko

p.s. loved the question!!!! Not sure I would want to be on the test flight however.

My problem with the plane question is that it states the plane is "sitting" on the runway but "moving". So which is it? I can't really see how it could be doing both.

sitting at time=0
moving, relative to the conveyer, but not necessarily the surrounding ground at all times after that, until such time as it achieves true flight.

locked wheels haveing nothing to do with flight.

the wheels of a plane are rotating freely.

how about this.

If the wheels, bearings, etc. were frictionless what do you think would happen if the converyor started to go backwards? The plane wouldn't move! The wheels spin freely. They are not like a cars wheels in gear. put a car on the conveyor and it would be stationary forever if the conveyor matched the wheel speed.

Basically as the plane went down the runway the converyor would be going faster and faster just spining the wheels faster and faster doing absoletely nothing to the plane.

Put friction into the equation and the plane will probably need a bit more runway.

Imagine.....a dinky car on a piece of paper. Put a small force on the car.......start pulling the paper. Pull the paper faster.....faster......faster nothing will happen. If the dinky car had trust it would move forward.

I am very worried if the scientific community can't wrap there head around this. To me it has very little to do with being a physicist and everything to do with common sense.

Keep in mind though.......and hopefully you see it. This seems to be something that people have trouble with because they are focusing on something else. I think it is that the wheels are locked to the plane somehow like a car in gear.

yes I like this question as well.

Joe

I am very worried if the scientific community can't wrap there head around this.I agree
Like I said, Google the question!!

Here's an easy one for everyone:
Is it possible to throw a ball into the air as hard as you can, have it stop in mid air, go a different direction, then have it accelerate back at you?

obviously throwing a ball in the air will cause it at some point to stop. Going another direction? Maybe with some wind?

Falling back down would constitute it going in another direction after it comes to a stop in midair.

Q, I get the plane answer now. Thanks. Although I agree with vmpv in that the sitting part makes the question intentionally misleading.

I'm glad to see you missed 99's ball question ;) Assumming Cyclism is correct as that is my answer too.

Cyclism is correct, but if I was marking this question on a exam, I would only give 4/5 marks. ;)
What about then have it accelerate back at you

It is implied in Cyclism's answer but not answered directly.

In a round room with a square door,....
There are 2 dogs in front of 2 other dogs.
There are 2 dogs behind 2 other dogs.
There are 2 dogs beside 2 other dogs.

How many dogs are in the room???