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  #26  
Old 03-06-2012, 03:16 PM
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Default Re: Science Things I Wish My Teacher had Told Me

I wish my high school physics teacher had helped me make a softer landing from the neat and tidy Newtonian equations for mass/energy/force/etc., that always gave me neat and tidy answers as long as I used the right formulas, to the fuzzier, more confusing stuff, like heat transfer. I went from liking physics to being bewildered and frightened by it, and that bastard was acting like the incomprehensible stuff he was babbling about was just as simple as calculating the momentum of a train. Who knows, maybe it is and I just didn't get it. But I distinctly remember it going from crystal clear in one chapter of the textbook to opaque in the next, and I gave up because I saw no way forward.
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  #27  
Old 03-06-2012, 03:48 PM
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Default Re: Science Things I Wish My Teacher had Told Me

I wish my teachers had gotten their information right, In HS physics the teacher was talking about the co-efficient of friction and made the statement that surface area was not a factor and that people who put wider tires on their cars for more traction were not gaining anything. At that point I knew he was wrong about that and was more skeptical about the other 'facts' he was teaching.
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  #28  
Old 03-11-2012, 10:18 AM
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Default Re: Science Things I Wish My Teacher had Told Me

Quote:
Originally Posted by thedoc View Post
I wish my teachers had gotten their information right, In HS physics the teacher was talking about the co-efficient of friction and made the statement that surface area was not a factor and that people who put wider tires on their cars for more traction were not gaining anything. At that point I knew he was wrong about that and was more skeptical about the other 'facts' he was teaching.
According to this MIT lecture, your teacher was correct. Though the professor acknowledges that it is counterintuitive, surface area does not affect the coefficient of friction. The coefficient of friction between a surface and object is found by taking the tangent of the angle where the surface needs to be tilted to in order for the object to slip. This is determined experimentally, and experimentally, objects of the same material will slip at the same angle even if they differ in surface area.

If you listen to the lecture, he actually assigns a homework problem in which the students must explain why racecars have wider tires even though it doesn't give better traction. I wouldn't have been able to guess the answer, but I checked the solution, which attributes the following explanation to The Flying Circus of Physics by Jearl Walker (I don't know if it's supposed to be a direct quote or if he just got the idea there):
Quote:
The frictional force on the tire does not depend on the surface area in contact with the pavement. Thus a wide slick is as effective as a narrow one. If the tires are spun over the surface as is often done in drag racing, then the wide [tires] have an advantage in that it has a larger surface area to heat and is less likely to melt. Melting greatly reduces the coefficient of friction.
You can find the solution set here, that particular one is 3.10.
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  #29  
Old 03-11-2012, 01:16 PM
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Default Re: Science Things I Wish My Teacher had Told Me

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Quote:
Originally Posted by thedoc View Post
I wish my teachers had gotten their information right, In HS physics the teacher was talking about the co-efficient of friction and made the statement that surface area was not a factor and that people who put wider tires on their cars for more traction were not gaining anything. At that point I knew he was wrong about that and was more skeptical about the other 'facts' he was teaching.
According to this MIT lecture, your teacher was correct. Though the professor acknowledges that it is counterintuitive, surface area does not affect the coefficient of friction. The coefficient of friction between a surface and object is found by taking the tangent of the angle where the surface needs to be tilted to in order for the object to slip. This is determined experimentally, and experimentally, objects of the same material will slip at the same angle even if they differ in surface area.

If you listen to the lecture, he actually assigns a homework problem in which the students must explain why racecars have wider tires even though it doesn't give better traction. I wouldn't have been able to guess the answer, but I checked the solution, which attributes the following explanation to The Flying Circus of Physics by Jearl Walker (I don't know if it's supposed to be a direct quote or if he just got the idea there):
Quote:
The frictional force on the tire does not depend on the surface area in contact with the pavement. Thus a wide slick is as effective as a narrow one. If the tires are spun over the surface as is often done in drag racing, then the wide [tires] have an advantage in that it has a larger surface area to heat and is less likely to melt. Melting greatly reduces the coefficient of friction.
You can find the solution set here, that particular one is 3.10.

Thanks I'll watch it when I have the time, but in reference to the second quote drag racing is a very specilized form of racing and I was thinking more of open wheel formula race cars. I'll be looking for current real world testing references in the lecture.
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  #30  
Old 03-11-2012, 08:24 PM
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Default Re: Science Things I Wish My Teacher had Told Me

Quote:
Originally Posted by Joshua Adams View Post
According to this MIT lecture, your teacher was correct. Though the professor acknowledges that it is counterintuitive, surface area does not affect the coefficient of friction. The coefficient of friction between a surface and object is found by taking the tangent of the angle where the surface needs to be tilted to in order for the object to slip.
[/QUOTE]

Interesting lecture and a good description of classical physics, and he is probably correct that the coefficient of friction does not change with an increase in contact area as tested in the lab. However a tire on an automobile is a dynamic system and is more complicated than just a simple block of rubber on a surface. What is incorrect is the statement that wider tires do not give better traction, in the real world wider tires do give better traction and if this is not due to the coefficient of friction then there are factors that scientists are not taking into account.
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  #31  
Old 03-11-2012, 11:11 PM
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Default Re: Science Things I Wish My Teacher had Told Me

The simple model of static friction is only an approximation. If you put different sized blocks of the same material on the same plate, then the simple model would predict that as the angle of the plate is steadily increased, all the blocks would begin to slide down the plate when a certain angle was reached.

This isn't what actually happens - the largest heaviest block will slide first with the smaller blocks hanging on till the angle is increased to a greater value.

The experiment can be repeated (if you have the right lab equipment) to eliminate static electricity effects, the lubricating effect of the air or other fluids and so on. Even with conductive metal blocks, all carefully cleaned and polished to the same surface finish and with the plate and blocks all contained within a vacuum, the smallest block still 'holds on' the longest.
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  #32  
Old 03-12-2012, 01:12 AM
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Default Re: Science Things I Wish My Teacher had Told Me

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Originally Posted by ceptimus View Post
Even with conductive metal blocks, all carefully cleaned and polished to the same surface finish and with the plate and blocks all contained within a vacuum, the smallest block still 'holds on' the longest.
Is that because the greater mass results in a greater force to overcome the friction?

I seem to remember this diagram. . . .

--J.D.
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  #33  
Old 03-12-2012, 11:47 AM
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Default Re: Science Things I Wish My Teacher had Told Me


The demo of the sliding blocks starts at 6:19 - click on that link to go straight to that position.

Last edited by ceptimus; 03-12-2012 at 12:02 PM.
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