# B Coefficient of Friction

1. Apr 7, 2017

### Blue Scallop

Let's say you two objects with weight of 2 kilograms but one is narrower and one wider. They are said to be similar coefficient of friction against a surface.. but what if the surface is rough? Won't the wider object has more friction?

Or still the same?

If still the same.. what if the surfaces of the wider object has more roughness compared to the surface of the narrower surface.. then it is no longer similar? And we can say the coefficient of friction of the rougher surface is more in value?

2. Apr 8, 2017

### Biker

It is just an approximation, Friction is really complex.
For a lot of objects and surfaces, Friction doesn't depend on area. You can maybe justify this by saying the pressure decreases because you increase the area and the area should increase the friction but these two cancel out and you have constant friction that doesn't depend on area

But If you really have a small area this assumption breaks down

3. Apr 8, 2017

### Blue Scallop

Can't surface roughness affect the friction? I think smooth surface has less friction.. any formula or principles of this?

4. Apr 8, 2017

### Biker

Absolutely, I was talking about the effect of area.

Roughness affects friction by changing the friction coefficient. So The more rough it is the higher its coefficient. For example Ice on Ice has a really small friction coefficient. Also, A seemingly smooth surface is not smooth at atomic scales so there are a lot of contributing factors in friction.

Other members will give you more detailed description if you would like

5. Apr 8, 2017

### sophiecentaur

'Roughness': imagine a pair of surfaces that consist of regular shallow grooves with vertical sides of very slippery substances. The grooves on each side could be arranged to coincide with each other and the faces would be locked, immovably, once the grooves engage. You would have to break the materials for movement to happen. "Coefficient of friction" would be infinite. However, if the grooves all had sloping sides, they would act like tiny 'inclined planes' and the ratio of the lateral force to the normal force could be independent of the actual load. The effective coefficient of friction would just be set by the angle of the slopes. (tan(θ))

6. Apr 12, 2017

### Jamison Lahman

Without delving too deep into the semantics, I believe they would have the same amount of friction (at least with regard to the surface. the friction due to the air may be different, but I'm unsure.) Any loss in energy would more accurately be described by collisions than friction.
It should be noted that the coefficient of friction should theoretically incorporate any 'roughness' of the surface.

7. Apr 13, 2017

### sophiecentaur

The Coefficient of Friction is just a convenient term which can be applied where the situation is linear. It can't work everywhere. The equation that we use is obviously only going to apply to an ideal situation. It is even more limited in a dynamic situation.