We were discussing the importance of friction i.e. positive and negative effects of friction and
classifications of friction with the help of our previous post.

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Now, we will be interested further to understand the coulomb's
law of dry friction with the help of this post i.e. define coulomb's law
of dry friction. We will find out here the coulomb's law of dry friction
and we will also introduce here the co-efficient of friction.

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**Define coulomb's
law of dry friction **

Let us see here now the coulomb's
law of dry friction. There will be basically three laws of coulomb's law of dry
friction. The first two laws, as mentioned below, was given by another
scientist whose name was Amonton.

Even the following first two laws was given by scientist
Amonton, but these laws also come under the coulomb’s law of dry friction.

We must note it here that all the three coulomb’s law
of dry friction are based on the experimental observations.

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*So, let us first see here the first law *

*So, let us first see here the first law*

According to the first law of coulomb’s law of dry
friction, magnitude of the frictional force will be directly proportional to
the normal load between the surfaces for a given pair of materials.

Where,

µ

_{s}is the co-efficient of static friction
N is the normal load

We must have to understand and note it here that the
co-efficient of friction will be always given for a pair of material and it
will never be given for a single material.

Co-efficient of static friction will be dependent only
on the two contacting surfaces.

On the basis of observation of experiments, it was noted
that co-efficient of static friction will be independent of normal load.

We have also used here one term i.e. magnitude of
frictional force in the first law of coulomb’s law of dry friction. Magnitude of
frictional force represent here the maximum static frictional force.

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*Let us see here now the second law *

*Let us see here now the second law*

According to the second law of coulomb’s law of dry
friction, magnitude of the frictional force will be independent of the area of
contacting surfaces i.e. apparent area for a given normal load.

We have again used here term i.e. magnitude of
frictional force and it represent here the maximum static frictional force.

On the basis of observation of experiments, it was noted
that co-efficient of static friction will be constant even when area of
contacting surfaces i.e. apparent area will be varied by a factor of 250.

If we assume two cases, let us think that there is larger
surface contact in first case and smaller surface contact in second case.

We will be thinking that the resistance offered due to
friction in these two cases will be different and resistance offered due to
friction in first case where there is larger surface contact of object will be
higher.

But, will it be higher?

No, co-efficient of static friction will be same in
above mentioned two cases and it was concluded after analyzing the observation
of experiments.

We must note it here that the above two laws of
coulomb’s law of dry friction are given for object which is in stationary
condition or in static condition and that’s why we have used terms like
co-efficient of static friction and maximum static frictional force.

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*Now let us see the third law of coulomb’s law
of dry friction *

*Now let us see the third law of coulomb’s law of dry friction*

Coulomb has given third law and according to the third
law of coulomb’s law of dry friction, magnitude of the frictional force will be
independent of the sliding velocity.

Co-efficient of dynamic friction will be nearly independent
of the sliding velocity.

Therefore, we have studied here the three laws of coulomb’s
law of dry friction. Further we will find out another concept in friction i.e. guidelines for solving friction problems in mechanics with the help of our next post.

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right hand side of page for further and continuous update from www.hkdivedi.com.

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**Reference: **

Engineering Mechanics, By Prof K. Ramesh

Image
courtesy: Google

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