Q.State the laws of rolling friction.

Rolling friction is the (small) resistive force on a body rolling over a surface; its laws describe how much smaller and how it depends on load, radius, area, and speed compared to sliding friction.

When a body such as a wheel or ball rolls over a surface instead of sliding, the resistance it experiences is called rolling friction. Its laws are:

1. Rolling friction is very much smaller than sliding (or static) friction for the same pair of surfaces and the same normal load — this is why wheels, ball bearings, and rollers are used to reduce friction in machines.
2. Rolling friction is directly proportional to the normal reaction ($N$) between the body and the surface, and inversely proportional to the radius ($r$) of the rolling body: $F_r = \mu_r \frac{N}{r}$ where $\mu_r$ is the coefficient of rolling friction. A larger wheel radius gives lower rolling friction.
3. Rolling friction is (nearly) independent of the area of contact between the rolling body and the surface.
4. Rolling friction is (nearly) independent of the speed of rolling, as long as the speed is not very large. …