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DEFINE COULOMB'S LAW OF DRY FRICTION

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.  

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.  

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. 

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. 

Co-efficient of static friction and static frictional force

Where, 
µs 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. 

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. 
Static frictional force

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. 

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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We will find out now the guidelines for solving friction problems in mechanics in our next post.  

Reference:  

Engineering Mechanics, By Prof K. Ramesh  
Image courtesy: Google    

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