In a flat collar pivot bearing, the moment due to friction is proportional to (left(r_{1} ight.) and

In a flat collar pivot bearing, the moment due to friction is proportional to \(\left(r_{1}\right.\) and \(r_{2}\) are the outer and inner radii respectively)

(a) \(\frac{r_{1}^{2}-r_{2}^{2}}{r_{1}-r_{2}}\)

(b) \(\frac{r_{1}^{2}-r_{2}^{2}}{r_{1}+r_{2}}\)

(c) \(\frac{r_{1}^{3}-r_{2}^{3}}{r_{1}^{2}-r_{2}^{2}}\)

(d) \(\frac{r_{1}^{3}-r_{2}^{3}}{r_{1}-r_{2}}\).