Equally important is the case when the object and the supporting surface are moving with some acceleration. You all have experienced the feeling of being heavier or lighter when an elevator starts or stops. In a large class of problems, we have to deal with objects that are either moved or restrained by means of a rope, a cable, or the like. I pull the same load horizonatally, on a frictionless surface, moving with constant velocity 3. Another type of force that has the same property of not being defined a priori, but to respond according to the counterforce applied is the force of friction .
Friction plays a fundamental role in everyday's life.
We can then say that friction grows in concert with the force applied, until it reaches a maximum value. For a given object, this maximum value depends on the properties of the two surfaces in contact. In other words, friction is proportional to the normal force . From this we see that is dimensionless. In fact friction and normal forces are orthogonal to each other .