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Online Physics Experts?
Old 07-07-2010, 12:15 PM   #1
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Online Physics Experts?

Hi all,

A few years ago I stumbled onto a website hosted by Physics Instructors/Professors. There, one can ask question. If the question is well asked and somewhat interesting, usually an answer would be provided.

Unfortunately, I can't recall the name of the site anymore. Have you seen it? If so, let me know its address.

Sam
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Old 07-07-2010, 12:32 PM   #2
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Gravity is overrated, that's my take

engineer by degree, seen more physics than I care to admit or think about LOL

difference between physicists and engineers is like difference between theoretical and reality.

In theory there is no difference between theory and reality.
In reality, there is a big difference
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Old 07-07-2010, 12:40 PM   #3
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A farmer, an engineer, and a physicist were all asked to build a chicken coop.

The farmer says, "Well, last time I had so many chickens and my coop was so and so big and this time I have this many chickens so I'll make it this much bigger and that oughtta work just fine."

The engineer tackles the problem by surverying, costing materials, reading up on chickens and their needs, writing down a bunch of equations to maximise chicken-to-cost ratio, taking into account the lay of the land and writing a computer program to solve.

The physicist looks at the problem and says, "Let's start by assuming spherical chickens....".
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Old 07-07-2010, 12:41 PM   #4
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A physicist, an engineer, and a statistician were out game hunting.

The engineer spied a bear in the distance, so they got a little closer. "Let me take the first shot!" said the engineer, who missed the bear by three metres to the left.

"You're incompetent! Let me try" insisted the physicist, who then proceeded to miss by three metres to the right.

"Ooh, we *got* him!!" said the statistician.
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Old 07-07-2010, 12:43 PM   #5
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A mathematician, a physicist and an engineer are each given $50 to measure the height of a building.

The mathematician buys a ruler and a sextant, and by determining the angle subtended by the building a certain distance away from the base, he establishes the height of the building.

The physicist buys a heavy ball and a stopwatch, climbs to the top of the building and drops the ball. By measuring the time it takes to hit the bottom, he establishes the height of the building.

The engineer puts $40 into his pocket. By slipping the doorman the other ten, he establishes the height of the building.
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Old 07-07-2010, 03:28 PM   #6
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Good ones. I have degrees in both physics and engineering so I can laugh at all the jokes

True Story: There was a post-Doc PhD physicist in our Physics department who left to go pursue a law degree with the hope of becoming a patent lawyer. During his third year of law school, he returned to the Physics department to say hi to everyone. Everyone asked how law school was going. He exclaimed that it was unbelievably easy. But everyone replied, how is that possible? He replied, all they do in law school is teach liberal arts majors to think like physicists.

Kramer
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Old 07-07-2010, 03:43 PM   #7
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I thought I had a few bookmarked, but could not find them. A google of:

"physics questions answered" brought up this near the top ( guess these are the low density results ):


Ask the Experts: Submit Question

and the link to their answer page:

Ask the Experts: All Answers

-ERD50
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Old 07-07-2010, 06:38 PM   #8
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Thank you ERD50 for the links.
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Old 07-11-2010, 03:52 PM   #9
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Well, it's been 4 days since I posted my question on that site. My question has not been answered. Worse, it has not been posted as a question either.

Since we apparently have a few physics major here, let's give it a try:

---------------------------------------------------------------------

1) Same rider, same road, same time, same high crosswind.

2) 2 identical motorcycles with one exception: 1st motorcycle has stock tires (130/70/17 rear, 110/70/17 front.) 2nd motorcyle has upgraded wider tires, same brand, same rubber compound, same thread, same shape (140/70/17 rear, 120/70/17 front.)

Question: Which motorcycle can be ridden faster (remember same rider) in crosswind. In other words: Do larger surface tire contact help at all in crosswind for a motorcycle.
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Old 07-11-2010, 04:04 PM   #10
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Strictly amateur here.

The sail area, rider+motorcycle remains the same. Tire contact patch irrelevant. Speed will be the same.

Rider's skill very relevant.

Empirical seat of the pants experience of 40+ years of riding various bikes.


edit 2 add: Just remembered from my EVOC driving class. Instructor demonstrating measurement of coefficient of friction with a weighted piece of tire.

Once it is determined. that number does not change for the same condition of pavement regardless of thread type, composition, width or contact patch area. At least for automotive applications.

He was a certified accident reconstruction expert who frequently testified in court.



By the way seems like homework.
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Old 07-12-2010, 07:56 AM   #11
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What ls99 said is my "opinion" too.
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Old 07-12-2010, 08:40 PM   #12
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Perhaps the answer isn't so much physics as mathematics:

Assuming both motorcycles and riders have the same performance characteristics, ie drag, terminal rpm, etc, it's going to be the drive tire diameter that determines top speed.

130/70/17 at 614mm (24.2") diameter takes 539 revolutions/km (335 per mile)
140/70/17 at 628mm (24.7") diameter takes 527 revolutions/km (327 per mile)

2.5% speed advantage to the slightly larger rear tire, because it's traveling an additional half-inch per revolution. ... the crosswind component is going to be neglible, the tire diameter differential is substantial. YMMV

Total Motorcycle Tire/Tyre Guide - Avon Roadrider
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Old 07-12-2010, 11:05 PM   #13
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I don't know about Westernskies mathematical approach. But from a physics standpoint the same kind of rubber on the same pavement should have the same coefficient of friction even if one is wider than the other (provided everything else is the same). Now, I know that in the real world that wider tires usually have better traction, but I believe that is because they are usually made from a much softer rubber compound. And I know that there is no room in the force friction formula for how wide a surface area the forces are spread, which I believe means that in your scenario you just wind up with the same overall force but with smaller force per unit measurements on the bike with the wider tire.

Personally, I think the wider tire would be a better choice in the scenario because it provides a wider base, theoretically a firmer side-wall, and definitely wider contact with a road surface that has at least some imperfections (so you're more likely to have at least some contact with good road surface with the wider tire).
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Old 07-15-2010, 10:50 AM   #14
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Originally Posted by Leonidas View Post
Personally, I think the wider tire would be a better choice in the scenario because it provides a wider base ...
I think so too. But I have not found a way to confirm it with theories and/or formulas.
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Old 07-15-2010, 10:55 AM   #15
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If it took a chicken-and-a-half a day-and-a-half to lay an egg-and-a-half, how many pancakes would it take to cover a rabbit?
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Old 07-15-2010, 11:07 AM   #16
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If it took a chicken-and-a-half a day-and-a-half to lay an egg-and-a-half, how many pancakes would it take to cover a rabbit?
Are they buttermilk, buckwheat, or blueberry pancakes?
Domestic or wild rabbit?
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Old 07-15-2010, 11:17 AM   #17
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Quote:
Originally Posted by Sam View Post
I think so too. But I have not found a way to confirm it with theories and/or formulas.
I don't think you have a "pure" physics question.

Quote:
1) Same rider, same road, same time, same high crosswind.

2) 2 identical motorcycles with one exception: 1st motorcycle has stock tires (130/70/17 rear, 110/70/17 front.) 2nd motorcyle has upgraded wider tires, same brand, same rubber compound, same thread, same shape (140/70/17 rear, 120/70/17 front.)

Question: Which motorcycle can be ridden faster (remember same rider) in crosswind. In other words: Do larger surface tire contact help at all in crosswind for a motorcycle.
edit - also, does this really change with speed of the cycle? A cycle will tip over in a crosswind standing still. Maybe going faster actually increases the stability, as the wheels are acting as gyros, and you have more inertial force. I think that needs to be determined first.

First, as Westernskies points out, the tires are not just wider they are different diameters. I think that if you want a "physics" answer, you need to phrase it as a "physics" question.

You seem to be making some assumptions in your approach:

Quote:
Do larger surface tire contact ....
Wider tires may have two different effects. Which one is more important to a crosswind?

1) they might give better traction, in which case they would be able to take a higher (crosswind) force before skidding out sideways. (Although, I seem to recall that in theory, tire width does not affect traction, as a wider tire has fewer PSI, and it all balances - in practice, this is not true because tires and roads are not perfect surfaces, this is why I say it might not be a physics question, but a real world question).

2) A wider tire would make the cycle less prone to 'tipping' - so if the major problem with a cross wind is that the force causes the cycle to tip over, then a wider tire may help, regardless of traction being better/worse/same.


So I think you have more of a 'real world' question, and the first thing I would want to know is whether tire 'grab' is the limiting factor in a cross wind force, or tire width (stability), or maybe something else?

Now that I re-read it, I'm not even sure what the question is. If the crosswind isn't actually pushing the cycle to some limit, I doubt the tire width has anything to do with speed. Would one cycle/tire combo be faster with no crosswind? I'm confused what you are looking for, and that may explain why you got no response from the physics forum.

-ERD50
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Old 07-15-2010, 11:40 AM   #18
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There is really only one way to definitively answer this question-

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