# B Maximum coefficient of friction

1. Apr 2, 2017

### FranzDiCoccio

This issue was the subject of this old post, now closed.
The question is: can a coefficient of friction (static) be larger than one?

I see confusing replies. Someone talks about glue, but I'm not convinced about that. It's a bit like wondering about the coefficient of friction of a wooden plank nailed to a wall.

I like the "micro hills" explanation by pixel01, and the table tennis video by rcgldr.
Actually, whenever something won't slide down a plane inclined at an angle larger than 45° it should be
$\mu_s>1$, right?
However I'm not sure about the table tennis rubber. To what extent the surface is really a plane? If it has grooves the object can hang onto those "mechanically" (I mean, at the macroscopic level).

Again, it is a bit like saying that the coefficient of friction between ice crampons and ice is very high.

I think that $\mu_s$ can be larger than one, but I'd like a more precise answer... I'm not sure there is one though. I guess that the surfaces one considers should not be too irregular, but I don't know whether a quantitative rule exists. When is a micro hill really micro? When it is small compared to the area of the sliding surfaces?

Thanks

2. Apr 2, 2017

### BvU

It's just a ratio, not an efficiency or something that otherwise has a restriction on the numerical value.

3. Apr 2, 2017

### FranzDiCoccio

Thanks BvU
I agree with that, but I still find the comments about glue confusing.

Is there a rule of thumb for understanding when it is a matter of friction and when it's not?

Is the force keeping an object from sliding on a plane is still friction, if the object is glued to the plane or if the surfaces are very irregular (on a scale comparable with the object size)?

4. Apr 2, 2017

### rumborak

I don't think it really works for glued/screwed etc contacts, since the formulaic implication of the coefficient is that the friction force is proportional to the normal force. That obviously is not the case for glue or screws. So you would be calculating a value with a formula that doesn't really apply.

5. Apr 3, 2017

### FranzDiCoccio

You're right!
Ok, what you're saying is that if I glue a block of balsa wood onto a plane, its resistance to sliding onto the plane has nothing to do with friction, which depends on the normal force, which is laughable for such a light material.

Cool, this definitely kills the glue argument, which, btw, never convinced me one bit.
I do not know why I did not think this myself, but really, thanks a lot!

6. Apr 3, 2017

### rcgldr

Table tennis racket rubber sheets have a coefficient of friction well above 1.0, more like 5.0+. Example windows movie video using a comb on a steeply angled racket until the comb slides off.

http://rcgldr.net/real/ttstick.wmv

7. Apr 4, 2017

8. Apr 7, 2017

### rcgldr

Top fuel and funny car dragster tires achieve a coefficient of friction around 4.5. The initial launch is close to 5 g's, but the exhaust from the engine is adding significant downforce. Race car tires are achieve 1.5 up to well over 2.0 (Formula 1 super soft tires).

As I posted previously, high end table tennis rubber has a coefficient of friction well over 5.0 with table tennis balls or other types of plastic.