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Friction is indispensable for our world.

Friction force (also referred simply as friction) plays the role of a dampener to the motion of a body that takes place in contact with another body. All motions involved in our daily life take place in contact with another body or medium. The force of opposition to the motion of a body in relation to another rigid body is called “friction” and in relation to fluid (liquid or gas) medium is called “drag”.

Friction is a huge disadvantage to us. A good part of the energy used in this world goes waste to counteract this force. On the other hand, this force is also responsible for our existence and the very existence of life on the planet. Imagine its absence. How would have we stopped once in motion? Everything would have been perpetually in motion with no control. Could this life have evolved without friction in the first place? Friction is simply indispensable to us.

Genesis of friction

When bodies come in contact with each other, large numbers of atoms of the two surfaces come close to each other, affecting a temporary joint (referred as cold welds in technical parlance) i.e. atoms from the two surfaces become part of one body mass. These joints result from electromagnetic force operating between atoms, when brought sufficiently close. The temporary joints between two surfaces inhibit relative movement between two surfaces.

In general two ordinary surfaces are uneven at microscopic level. The surface is actually formed of small hills and valleys. All points across the surfaces do not come in contact. Still, there are large numbers of such contact points, forming temporary joints.

Surfaces in contact

The surface is actually formed of small hills and valleys.

The weight of the body plays important role in determining friction. When the overlying body has greater mass, it applies greater force at the interface. More points come in contact or become sufficiently close to form joints. Further, all contact points are not cold welded or joints. Due to the weight of the overlying body, more of the contact points become temporary joints or become stronger joints, requiring larger external force to initiate motion. Thus, friction depends on (i) numbers of points in contact (nature of the surfaces in contact) and (ii) normal force at the contact surface which presses them to come closure.

Generally, a smooth polished surface is known to offer smaller friction with respect to a rough surface. When the surface is smooth, then there are more contact points, but corresponding force per point is smaller. In this case, the weight of the block, pressing against the surface beneath, is distributed across larger numbers of contact points. The net result is that there are greater numbers of contact points, but fewer welded joints opposing motion. Thus, a smooth surface offers smaller friction in comparison to rough surface.

However, if the surfaces are genuinely smooth to perfection and brought together, then there are much greater numbers of contact points, which are already sufficiently close and produce still larger numbers of weld sites. In such case, two bodies become almost inseparable and require a much greater external force to separate two bodies. If the joint is done between very smooth surfaces in the absence of air i.e. vacuum, then the cold welding, at contact sites covering larger contact area, makes the two pieces as one and the bodies are mechanically inseparable.

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Source:  OpenStax, Physics for k-12. OpenStax CNX. Sep 07, 2009 Download for free at http://cnx.org/content/col10322/1.175
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