Friction - Influence of mass with elastic materials

[YeeHaa]

Hi,

I'm doing some research on friction, and I did some tests with 4 different types of floor.
Now the first of these four floors is a sport surface (it has a polyurethane top coat), the second one is a floor-tile (with fabric on top), the third one a piece of foam and the fourth a piece of wood.

All of these materials were tested against metal.

Now I tested every floor with 3 different weights, and took some measurements (measured the normal force and friction force).
When I look at the Coefficient of Friction, it seems that for the first two floors (sport surface and floor tile), the Coefficient of Friction is LARGER when the weight is SMALLER. As soon as I increase the weight (2nd and 3rd weight), the Coefficient of Friction stays the same (it's the same for both weights).

Could this have to do with Amonton's Second Law? Maybe less weight means the surface deformes little.. (but in my eyes this would mean that friction should be lower), and with a lot of weight more of the surface at microscopic level touches the metal?

Some numbers:
Sport surface - metal:
Weight 1: (about 9 kg): CoF = 0.27
Weight 2: (about 15 kg): CoF = 0.19 (+/-)
Weight 3: (about 25 kg): CoF = 0.19 (+/-)

Floor tile - metal (same as sport surface, guess that's coincidence)
Weight 1: (about 9 kg): CoF = 0.27
Weight 2: (about 15 kg): CoF = 0.19 (+/-)
Weight 3: (about 25 kg): CoF = 0.19 (+/-)

Foam - metal:
Weight 1: (about 9 kg): CoF = 0.42
Weight 2: (about 15 kg): CoF = 0.43 (+/-)
Weight 3: (about 25 kg): CoF = 0.41 (+/-)

Wood - metal:
Weight 1: (about 9 kg): CoF = 0.11
Weight 2: (about 15 kg): CoF = 0.12(+/-)
Weight 3: (about 25 kg): CoF = 0.11(+/-)

 

[YeeHaa]

Anyone?

 

[AlephZero]

You haven't given enough about the experimental setup to even speculate about the cause. Some basic missing facts are

* Were you measuring static or dynamic friction?
* What was the geometry of the moving object and how was it loaded with weights? (we don't know if the pressure between the surfaces was uniform across the whole contact area or not).
* How did you apply the force to overcome friction? (i.e. where did you apply the force to the object and were you pushing or pulling? Is it possible it was trying to tip over and "dig into" the floor before it started moving?

But there's no guarantee you will get any useful answers even with that information. The experiment is probably too uncontrolled to "explain" anything in much detail. When you say "a sports floor with a polyurethane top coat" we don't know whether it is newly laid or 10 years old, how smooth it is, how clean it is, whether it has been polished with anything that might act as a lubricant or an adhesive, etc, etc...

If you really want to figure out "exactly" what is happening, you have to control all those types of factors, and/or vary them to investigate what effect they might have.