How do we measure friction?

Friction is the force that prevents one solid object from rolling or sliding over another. Although frictional forces, like the traction required to walk without slipping, may be advantageous, they also pose a significant amount of resistance to motion. About 20 percent of the engine power of automobiles is consumed in overcoming frictional forces in the moving parts.
measurement by coefficient of friction:
The friction of sliding solids can be described by two straightforward experimental facts. First, there is a close relationship between the area of contact and the degree of friction. The frictional force experienced when pulling a brick along a table is the same whether the brick is flat on the table or standing on end. Second, friction is proportional to the load or weight that presses the surfaces together. If a pile of three bricks is pulled along a table, the friction is three times greater than if one brick is pulled. Thus, the ratio of friction F to load L is constant. This constant ratio is called the coefficient of friction and is usually symbolized by the Greek letter mu ($\mathrm{\mu <!--\; \mu \; -->}$). Mathematically, $\mathrm{\mu <!--\; \mu \; -->}=F/L$. The coefficient of friction is dimensionless since both friction and load are measured in units of force (such as pounds or newtons). The coefficient of friction for one or more bricks sliding on a clean wooden table is around 0.5, which means that just to overcome friction and keep the bricks moving along at a constant speed, a force equal to half the weight of the bricks is needed. The frictional force itself is directed oppositely to the motion of the object. Because the friction thus far described arises between surfaces in relative motion, it is called kinetic friction.