How can friction make a car move, turn, and stop?

[DaveC426913]

...it is gravitational forces that are responsible for friction.

Not necessarily. There has to be some normal force in order for there to be a friction force, yes. But the normal force doesn't have to be provided by gravity.

Not at all. Place your hands out in front of your like you're praying and them rub them vigorously. Friction without any need of gravitation.

In jzz's defense, it appears they were speciifically addressing the starting, stopping and turning of a vehicle, rasied by the OP, not generalized friction.

(My non-gravity example, before I realized this, was going to be screws, nails and knots; all of which would fall apart if not for friction.)

 

[jzz]

No, you are the one who made the claim, so you provide a reference that backs it up.

Guillame Amonton's Second law of friction:
The force of friction is independent of the apparent area of contact.

 

[PeterDonis]

Guillame Anton's Second law of friction:
The force of friction is independent of the apparent area of contact.

This isn't a reference, since there is no link or source given.

 

[jzz]

This isn't a reference, since there is no link or source given.

I won't argue semantics with you, he is a historically well known and documented scientist.

 

[PeterDonis]

I won't argue semantics with you

I'm not "arguing semantics". I'm telling you the PF rules for references, which you signed up to when you became a member.

he is a historically well known and documented scientist.

So what? You still need to give a specific link or source (as in, what book or paper, published where and when). You could be quoting (or claiming to quote) Einstein and you would still need to give a specific link or source.

 

[sophiecentaur]

So what? You still need to give a specific link or source (as in, what book or paper, published where and when). You could be quoting (or claiming to quote) Einstein and you would still need to give a specific link or source.

That's a bit OTT, surely. I would quote you from my O level Physics book if I still had it. Would you want references to N1, N2 and N3 every time they're quoted? Are you really not familiar with the priniple?

 

[jzz]

So what? You still need to give a specific link or source (as in, what book or paper, published where and when). You could be quoting (or claiming to quote) Einstein and you would still need to give a specific link or source.

Sorry, my bad. I didn't know that PF rules required a reference. Will a Wikipedia reference do as a reference?

 

[PeterDonis]

Will a Wikipedia reference do as a reference?

If it has appropriate sources cited (which most Wikipedia articles on scientific topics do), yes.

 

[PeterDonis]

Are you really not familiar with the priniple?

In an "I" level thread, it cannot be assumed that all thread participants would be familiar with it, no.

In any case, a specific named law was cited, which makes it a more specific claim than just the general principle. That would require a reference even if the general principle were common knowledge.

 

[jzz]

https://en.m.wikipedia.org/wiki/Fri...t Law: The force,the apparent area of contact.

 

[phinds]

Your refernce does indeed include "Amontons' Second Law: The force of friction is independent of the apparent area of contact." but that doesn't make it true and in fact, taken as a stand-alone statement it is nonsense. As I pointed out earlier, it is ONLY true if the normal force per unit area remains unchanged. That "Second Law" needs to be paired with "Amontons' First Law: The force of friction is directly proportional to the applied load."

 

[jzz]

Your refernce does indeed include "Amontons' Second Law: The force of friction is independent of the apparent area of contact." but that doesn't make it true and in fact, taken as a stand-alone statement it is nonsense. As I pointed out earlier, it is ONLY true if the normal force per unit area remains unchanged. That "Second Law" needs to be paired with "Amontons' First Law: The force of friction is directly proportional to the applied load."

Point taken!

 

[PeterDonis]

Your refernce does indeed include "Amontons' Second Law:

As well as Amontons' First Law and Couloumb's Law of Friction. Unfortunately, nothing in that section is footnoted so it is not clear where the statements of the laws that are given are coming from. This is one of the pitfalls of using Wikipedia as a reference; there are plenty of references given in the article, but which, if any, of them, contain those laws of friction and a discussion of their meaning and applicability? That's why I qualified my previous post about Wikipedia with "if it has appropriate sources cited". Those statements in the article linked to do not.

 

[Ibix]

Interestingly, the Wiki article on Amontons himself also lists the laws and cites a Washington.edu page which does appear to have some references, and also some discussion of the (major) limitations of those laws.

 

[jbriggs444]

Amonton's law of friction is not a law of physics generating exact results. It is an engineering approximation that usually generates good approximate results. The coefficient of friction can vary with contact pressure.

See, for instance, https://www.nature.com/articles/s41598-023-29764-w

It is my understanding that high performance tire designers are well aware of this.

 

[sophiecentaur]

Amonton's law of friction is not a law of physics generating exact results. It is an engineering approximation that usually generates good approximate results. The coefficient of friction can vary with contact pressure.

See, for instance, https://www.nature.com/articles/s41598-023-29764-w

It is my understanding that high performance tire designers are well aware of this.

It's one of those laws that we learned in school and it was counter-intuitive for a start. Then you read the 'proof' which involves the contact pressure remaining constant due to surface deformation. That sort of makes sense until you see racing cars and dragsters with slicks.

Finally, PF takes up the cudgel and we're chewing it over again.