Since pressure equals force divided by the area of contact, it works out that the increase in friction generating area is exactly offset by the reduction in pressure; the resulting frictional forces, then, are dependent only on the frictional coefficient of the materials and the FORCE holding them together. Since friction means a contact force, the nature of contact will influence friction force. What could that be? How it does not depend on area of contact. This means that even if you have two heavy objects of the same mass, where one is half as long and twice as high as the other one, they still experience the same frictional force when you drag them over the ground. It does not depend the area of contact, it doesn't depend on the sliding speed. $\begingroup$ @Daniel Griscom : it is not home work I'm just going through some text related to friction.

$\endgroup$ – Srinivas Rathikrindi Feb 10 '16 at 3:03 Or if this does not exist, what is the proof that contact area does not affect friction force? This device consists of a number z of disc pair, one connected to the input shaft and the other disc from the pair connected to the output shaft. Smoothness of objects affect the friction force. Notice that both of these friction formulas ONLY depend on the coefficient of friction and the normal force. EDIT: So, the book is about designing a friction clutch. my question is If friction is result of surface contact of two objects. Depending on the pair of surfaces involved, they may find that contact area does or does not have an affect on the amount of friction that is generated, depending of the relative importance of molecular attractions between the particular surfaces chosen. Read 19 answers by scientists with 43 recommendations from their colleagues to the question asked by Prabhakar Purushothaman on Oct 10, 2014 The force due to friction is generally independent of the contact area between the two surfaces.