Friction in Physics static and kinetic 

Friction in Physics is important topic as we have already studies  about many forces like normal force, tension force, spring force gravitational force in previous post friction is also same type of force but this is new type of force we will study about new type friction force in this topic you can refer Newton's law of motion   .

Friction force is more complicated  than other forces so it required more attention and concepts friction force is of two types one is static friction and other is kinetic friction first we will see kinetic friction and then static friction see picture below for visualization.

                                               

   from the above picture it is clear that friction force acts between two surfaces which is shown in the figure between two surfaces but question arises why friction force acts between two surfaces ?answer when you will see with microscope surfaces you will find small holes in both the surfaces it will look like rough surfaces normally you will see smooth but actually it is not smooth now when block moves with velocity v then block small holes trap in other surface small holes and velocity of block decreases and hence a kinetic friction force acts upon the block in opposite direction of block motion hence we can say "friction acts due to roughness of the surface".
value of kinetic friction from above FBD of block fr = 𝛍N where 
normal reaction force and 𝛍 is coefficient of friction N = mg hence fr = friction force = 𝛍mg, 𝛍 value is constant its value depends upon the roughness of the surface if roughness is more then value of 𝛍 will be more and if roughness is less then value of 𝛍 will be small for example you have played carrom board in childhood we use powder on carrom board surface to reduce roughness of the surface for making smooth surface  then friction force value is reduce see below some question.
                                         

    from above picture question 1 we will calculate the value of friction we know that friction force fr = 𝛍N = 𝛍mg = 0.5*5*10 = 25N now from question 2 here 10N force is also acting in downward direction so first calculate normal reaction force write the vertical force equation mg force will be vertical downward and normal will be upward hence N = 10+mg = 10+5*10 = 60N now use friction force formula fr = 𝛍N = 0.5*60 = 30N hence friction force is 30N in this case compare question 1 and question 2 value in first case 25N friction force but in question 2 friction force value is 30N why ? answer in second case 10N more force pushing block downward hence the small holes between two surface will trap more so its friction force value will be more this is physical significant.




now for question 3 we can solve similarly taking vertical force and horizontal force suppose block is moving in right direction with acceleration a here you have to take 100N force component in both direction vertical and horizontal now breaking the component horizontal component will be 100cos37 and vertical component will be 100sin37 now we can balance vertical component 
N+100sin37  = mg or N = mg -100sin37 
now horizontal equation 100cos37 - fr = ma or 100cos37-𝛍N =ma now put the value of N and m we will get value of acceleration easily.
Static friction force is define as tendency of relative motion its mean that motion is just about to start it also mean body is at rest no motion suppose a heavy mass is laying on a ground two children of your colony are trying to move it after applying by two children total force the heavy mass is not moving then where does this force gone of course this force applied by children is adjusted by friction force which is not seen again two more children come and join this group to move this heavy mass but still it is not moving again some children come and join this group to move this heavy mass suppose 10 children join and applied force then heavy mass just start to move this is the limiting force adjusted by heavy mass and this is called static friction hence at rest friction force applied is called static friction force but limiting.
friction force acting during body motion is called kinetic friction force kinetic friction force value is less than static friction force i hope that you have enjoyed learning Friction in Physics static and kinetic thanks for reading.
dated 19th Aug 2018