In my experiments, on a flat plane, when I incrementally increase the normal force acting on a rubber block, I am measuring a decrease in the static coefficient of friction. I am also measuring the friction force, which increases as normal force increases BUT this is not a proportional increase, hence the drop in coefficient of friction value. For instance Mass: 5kg, 10kg, 15kg, 20kg, COF: 1.8, 1.6, 1.4, 1.3 Why is this? Shouldn't the static coefficient of friction increase with normal force? Or is the roughness of the surface sample I am using playing a role?