# The coefficient of friction

1. Feb 19, 2016

### Nick tringali

Hello physics forums,
I recently carried out an experiment testing the coefficient of friction of different grit sandpaper
My data supports that when you raise grit you decrease the cof.
My problem is WHY, why does the size of the grit have any impact on the coefficient of friction. Any ideas would help.

2. Feb 19, 2016

### Staff: Mentor

To get the ball rolling, what really your ideas? How does sandpaper create friction? Does that mechanism change with the grit?

3. Feb 19, 2016

### Nick tringali

Ok so when the grit is around 2000 it feels like paper but when the grit is 50 it feels rough so here are some ideas
-There is more space between the particles in the 50 grit (but why does that make a difference physicaly
-There is more penatration into the surfaces
Thats all i got

4. Feb 19, 2016

### Nick tringali

Sand paper is rough but..... Why does rough raise the cof is what im geting at

5. Feb 19, 2016

### CWatters

Imagine you had two sheets of very coarse sandpaper with the rough surfaces together. Try sketching an enlarged cross section. What would have to happen for them to slide over each other?

6. Feb 19, 2016

### Nick tringali

The grits work bump into eachother

7. Feb 19, 2016

### Stuart21

Thats easy. As the roughness increases, the number of points of contact / area will reduce.

Stiction per point of contact, x reduced number of contacts / area = ∆µ < 0

Last edited: Feb 20, 2016
8. Feb 19, 2016

### Nick tringali

Thanks bud

9. Feb 21, 2016

### Nick tringali

Are there any formulas with mu and number of contact points?

10. Feb 21, 2016

### sophiecentaur

I don't think COF is a valid / useful quantity for all surfaces. The reason that COF is a useful quantity in many cases can be explained by the change in area of contact between faces in contact if the normal force changes. If you are dealing with a pair of surfaces which don't follow Hooke's Law then more coefficients are needed than COF to describe what happens. Sandpaper is a bit like two sets of gear teeth and the friction there is very non-linear (infinite until the teeth actually break).

11. Feb 21, 2016

### Nick tringali

Thanks so there are no related formulas that come to mind?

12. Feb 21, 2016

### sophiecentaur

Either use the old favourite one or, when that fails, you have to get down to specifics of each surface profile, I would think. I wouldn't look for a "formula", as such.