Coulomb suggested that for variable loads W the ratio P/W is constant for given materials and a given geometry(r= constant). Looking at eq.(2) we see that a must be a constant for given geometry and materials. Coulomb added that, for given materials and variable radius, theratio P/W varies inversly as r; that is, as the radius of the cylinder is increased, the resistance to uniform motion for a given load W decreases. Thus, considering Eq. (2) again, we may conclude that, for given materials, a is also constant for all sizes of rollers and loads. However, other investigators have contradicted this. According to them, coefficient of rolling resistance is dependent on radius of the wheel and loads besides many other factors. We leave the discussion on rolling friction at this stage and will discuss more while studying dynamics.