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Quick question - help pls
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flint
science forum beginner


Joined: 11 Jun 2005
Posts: 5

PostPosted: Sat Jun 11, 2005 7:22 am    Post subject: Quick question - help pls Reply with quote
Hi all, I have an urgent query about static friction. I know that the
friction force is the sum of the normal force x the friction
coefficient. Is the area of the brake pad irrelevant?

By way of example, would the braking force of a pad 10mm x 10mm under
force of say 100N be the same as a pad 20mm x 20 mm with the same 100N
force?

Is the area of the friction pad simply related to wear rate?

Thanks in advance.
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flint
science forum beginner


Joined: 11 Jun 2005
Posts: 5

PostPosted: Sat Jun 11, 2005 10:34 am    Post subject: Re: Quick question - help pls Reply with quote
Thanks for your reply Claudio. Much appreciated. Comment from others
is still very welcome.
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Claudio Grondi
science forum beginner


Joined: 23 May 2005
Posts: 38

PostPosted: Sat Jun 11, 2005 12:11 pm    Post subject: Re: Quick question - help pls Reply with quote
Quote:
Hi all, I have an urgent query about static friction.

So here an urgent reply:

as I can remember I had learned at school, that
the size of the area of contact doesn't matter, so
same coefficient can be used.

Working with rubber and looking at the lateral
accelerations and break forces occuring e.g.
while driving a Formula 1 racing cars
(lateral accelerations and braking forces
four times the weight of the car) shows, that
the assumption above doesn't always apply,
e.g. in case of such inhomegenous material
like rubber and especially in the situation it is
formed to a tire. Here the friction coefficient is
highly dependent on the size of the contact area
for same weight (or vertical force).

As I mean to know about the state-of-the-art
the phenomenon of friction is not yet very well
understood, so you are good advised to
stick your way of calculating friction to the
examples given along with the values of the
friction coefficients not trying to generalize
the results for all other materials or ways
of experiencing the effects of friction (e.g.
ways of measuring it).

Quote:
Is the area of the friction pad simply related to wear rate?
As I know, there exist empirical formulas to

calculate wear, but they are valid strictly
in the range of conditions they were derived
from, so there is no simple relation.
The phenomenon of wear is also not yet very
well understood, so you are forced to stick
to the conditions for which formulas exist and
can't generalize them for all materials and
conditions.

Because I am not an expert in this area,
maybe someone else can provide more
detailed view on the state-of-the-art in
the areas of friction and wear?

Claudio

"flint" <flint@f2s.com> schrieb im Newsbeitrag
news:1118481765.987892.255220@g47g2000cwa.googlegroups.com...
Quote:
Hi all, I have an urgent query about static friction. I know that the
friction force is the sum of the normal force x the friction
coefficient. Is the area of the brake pad irrelevant?

By way of example, would the braking force of a pad 10mm x 10mm under
force of say 100N be the same as a pad 20mm x 20 mm with the same 100N
force?

Is the area of the friction pad simply related to wear rate?

Thanks in advance.
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N:dlzc D:aol T:com (dlzc)
science forum Guru


Joined: 25 Mar 2005
Posts: 2835

PostPosted: Sat Jun 11, 2005 12:24 pm    Post subject: Re: Quick question - help pls Reply with quote
Dear flint:

"flint" <flint@f2s.com> wrote in message
news:1118481765.987892.255220@g47g2000cwa.googlegroups.com...
Quote:
Hi all, I have an urgent query about static friction.
I know that the friction force is the sum

*product*, not sum...

Quote:
of the normal force x the friction
coefficient. Is the area of the brake pad irrelevant?

Unless you get very small or very large area, yes. But it is
possible that contact forces can affect the coefficient of static
friction. This is not a number that is as universal as the speed
of light.

Quote:
By way of example, would the braking force of a
pad 10mm x 10mm under force of say 100N be
the same as a pad 20mm x 20 mm with the
same 100N force?

Be really close, yes. Within a percent difference or less, most
likely.

Quote:
Is the area of the friction pad simply related
to wear rate?

And heat disspation area. The bigger the surface is, the better
the heat transfer to the environment of the "lost" energy.

David A. Smith
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