Converted

Just because you can't wrap your head around it... lol

It's ok, the majority of you CAN'T... join the club.

Hojo061782

Apparently you can't read because I was answering your new question...

Let me say this as simply as I can...

WHEEL SPEED IS MEASURED IN ANGULAR UNITS!!!

What does this mean? You have to pick your point of reference for the linear velocity! Where the hell is that going to be? Point of contact? Top of the wheel?

Quote:

Originally Posted by you said

What does this mean? If the wheel is moving forward at 5 or 50MPH, what is the point of reference? Are we talking where the wheel hits the ground? Or are we talking the top of the wheel? Don't talk all condescending to me if you can't understand that there's a difference...

Hojo061782

Or are you talking about axle speed? If you're talking about axle speed, well the axle is connected to the plane, so it travels at the same speed that the plane does...

dom

<---Not getting it.

You said yourself,

Converted

Obviously measuring wheel speed as it hit's the ground.... as the treadmill matches it, it doesn't matter WHAT the propulsion method it, it could having fucking rockets on the back of it... lol

BUT!

What would happen, is the wheels would gain so much speed, so fast... that the bearing would fail before any sort of lift ever took place.

Converted

Because I was refering to the original (aka mythbusters) problem... in which case that's what I meant. If you're just looking at the PLANES speed, then you can match whatever you want, and the wheels will just spin at incredible speeds...

Hojo061782

Read my previous posts...did I ever mention any sort of propulsion? Geez...talk about bad reading comprehension.

Okay, so if we're talking about the speed of the wheel at the point that it hits the ground, then the treadmill would be at a standstill, since the wheel's linear velocity at that point would be zero. This is the definition of a wheel that doesn't slip.

Converted

Oh my god... you're just completely not getting it...

I've learned over the years that the smarter somebody thinks they are... the more they complicate EVERYTHING, and sound even more stupid in the end.

This is a simple equation, and you're throwing in all sorts of unneccesary crap...

A wheel spinning at 50MPH is just that.... A WHEEL SPINNING AT 50MPH... obviously the size of the wheel may make the RPM required to hit 50MPH different... but this is assuming that's already taken into account.

Just answer this question for me:

A wheel is connected to an axle is on a treadmill, and the axle is BOLTED to the treadmill.... If the wheel is measured to be spinning at a rate of 40MPH... how fast is the treadmill moving?

Hojo061782

This has nothing to do with me being smarter than you or me overcomplicating things or anything. This just has to do with simple physics.

A wheel doesn't "spin" at 50MPH. It may spin at 50 radians per second, but it doesn't spin in the units of miles per hour. A wheel as a whole may travel at 50MPH relative to the ground. However, when it does so, the top of the wheel is moving at 100MPH, while the bottom of the wheel (the part in contact with the ground) is moving at 0MPH.

The problem is that you're confusing forward motion of the wheel with the spinning of the wheel, which are two completely different things, and really have no relationship unless the wheel is in contact with the ground.

So, in your scenario, lets say that the wheel as a whole is travelling at 50MPH, and is in contact with the ground. That means that the linear velocity of the wheel at the top of the wheel is 100MPH, but in contact with the ground, the velocity will be 0MPH. If this is on a treadmill that measures the velocity of the wheel in contact with the ground, then the treadmill will not move.

Converted

So.... say I'm driving my TL on the interstate...

You're saying that if I'm driving at 50MPH, the TOP of my wheel is actually going 100MPH? lol

If you believe that, then I think I'm done trying to reason with you.

Hojo061782

I know that...and it's physically demonstrable (instead of just being a mathematical construct).


<applet code="freeRolling.class" codebase="http://www.phy.ntnu.edu.tw/java/FreeRolling/" height="360" width="550">









</applet>

If the above applet doesn't appear, take a look at this website. You'll need Java for it to work.

Watch the applet and pay particular attention to the lower portion of the applet. The white vector shows the direction and speed of the wheel as a whole as it moves. The red vector shows the linear velocity of a point on the wheel with respect to itself. When you add the red and white vectors, you get the grey vector as the result. The grey vector is the linear velocity of the point on the wheel with respect to the surface of travel. You'll notice that the vector is twice as long as the speed of the wheel as a whole at the top, while the vector has zero length at the point of contact.

That's my whole point. If you're measuring the speed of the point of the wheel that is in contact with the ground with respect to the ground, you're always going to measure a velocity of zero.

Converted

Well, obviously... if unless you're doing a burnout...

There's also a measurable wheel-speed rotation... whether you measure in RPM's, Feet/per second, or miles per hour...

If you know the circumference of a wheel, you can easily measure how many feet per revolution it can roll... with that number, you can easily determine how many MPH the wheel is travelling, given a certain RPM...

So this is all assuming the "MPH" in reference has been determined based on the wheels RPM and size... RPM is always RPM, there's no variables...

When I say a wheel is travelling at 50MPH, that obviously assumes that "would be" the speed of the center of the wheel, given a solid contact patch without slipping... besides the fact it's on a treadmill, or spinning in the OPEN AIR, you can still easily measure MPH of a wheel, by it's RPM.

So why have you over-complicated this so much?

A treadmill MATCHING the WHEEL SPEED of a plane, will cause the plane to go nowhere, regardless of it's propulsion... would you not agree? In order for the plane to have forward movement, that would assume that the wheel is moving faster than the treadmill, but given the problem, we know that's not possible... because the treadmill will continue to match it...

Like I said, the bearing would fail before any other result occured...

Hojo061782

Since you've qualified your statement, by saying the above, I would still disagree. For this reason:

If the center of the wheel is travelling at 50MPH, then the axle is travelling at 50MPH and the axle connected to the plane, so the plane must be travelling at 50MPH. Therefore, using your terminology, wheel speed and plane speed are indisitinguishable.

And as you've already said, if the treadmill matches plane speed, then the plane will take off. Well, since that's the same as your term for wheel speed (as shown above), then the plane will take off.

Converted

Well, now you're just twisting crap around, because you're making me attempt to explain it in your B.S. terms...

And you're wrong, because the technical "ground" is the treadmill, which the plane IS travelling at 50MPH on, you're correct... but in relation to it's surroudings and the air that is NOT on a treadmill, it's NOT moving, producing no airflow... so I'm still correct.

I also said that the speed can be measured using RPM, which has NOTHING to do with the axle... so qualify THAT...

Your attempt at looking really smart, just shows that all your "schooling" has removed the area of the brain known as "common sense".

Hojo061782

The problem is that common sense and intuition lead to incorrect answers here. I have plenty of common sense...I just know where to apply it and this isn't the time to apply it.

Basically, here's the deal. Let's assume infinite strength of the individual components and no friction between the axle and the wheel. With those assumptions, essentially your supposition results in the treadmill moving at an infinite speed (because as the plane speeds up, the treadmill speeds up).

However, with the assumptions I make above, the wheels spin at an infinite rate (which is OK because they're really, really strong in this case ), and since the plane has no friction with respect to the ground (because there's no friction with respect to the axle), it still moves forward and eventually generates the lift to take off.

Now if you're going to say something like, "well you can't make such ridiculous assumptions about the strength of the bearings, etc.", keep in mind that you're making assumptions that the treadmill will continue working, etc., which is just as ridiculous.

Ultimately, the problem is that people use heuristics (read: common sense) to determine the answers to questions. Unfortunately while heuristics will often lead to correct answers in common situations, in the uncommon situations (such as this), heuristics break down and lead to incorrect answers. So then it behooves a person to have a good threshold for when common sense should be thrown out the window.

Converted

It sounds like you're finally agreeing with what I was saying....

In that it's an impossible situation, based on the fact that no equipment would be able to support such a test...

And I'm glad that you are able to see the difference between this, and the original (mythbusters) situation...

Hojo061782

Given your ultimate description, I'll there's a "qualified" agreement.

It just seems, though, that your situation requires so many hypotheticals that analysis becomes rather useless. Though that's my own bent, I suppose. To each his own, eh?

Oh, and apologies on the name-calling. :gheyhug:

Converted

Well, as a member of other forums (NASIOC OT) we've had this debate before... and it gets frustrating when people just "dont get it" period... especially considering the original (mythbusters) situation... it's so damn easy to understand, when people don't get it and argue about it, it becomes tiresome... lol

I think 99% of the people are on the same page now?

dom

So have you two decided if whiskers is gay yet?

Hojo061782

Definitely...his bearings would definitely fail if went at such speeds on a treadmill...

Mizouse

dammit just this thread already!

The Dougler

iTrader: (1)

zeroday

So if the plane LANDS on a tread mill that's going twice the speed of the plane in the same direction as the plane, can the plane come to a stop?

Converted

If it's a propeller driven plane like a Cessna, then no... because it uses the brakes to stop...

A jet has reverse-thrust capability... so yes, it would!!

zeroday

I was kidding but thanks for the insight.

Converted

I know... but I figured since I had the answer, why the hell not... lol

jlukja

I'm still confused. Which is gayer on a treadmill, Whiskers or dom?

svtmike

Whiskers is gayer over here.
Whiskers is gayer over there.
Whiskers is gayer everywhere.

srika

what if a JET lands on a SHEET OF ICE - can it stop? the wheels are skidding and have zero traction.

Converted

They actually DO this sometimes... like I said above, jets can use reverse thrust to stop, so yes.

srika

yea i know. wouldn't this also help us answer the initial question?

srika

ps. only reason im posting in here is cuz im bored

Converted

Well, the initial question is true for ANY plane... but landing is different because some planes truly need the wheels to stop, unlike some jets that can use reverse thrust.

srika

true