# Static friction force

Friction force is of two types one is kinetic and other is static in this topic we will study about static friction force we have already learn in previous post about kinetic friction force you can refer for basic concept about friction force here  actually kinetic friction force is easy to understand but static friction force required concept to understand its rule student generally confuse in static friction force don’t worry it is not so hard you can understand this after discussion of concept of static friction we will study here every concept and question for static friction force which will clear your all doubt you will learn here static friction force hidden concept.

Static friction force important point
Static friction force apply when body will be at rest when there will be tendency of relative motion actually body at rest no relative motion started, static friction force value is variable it is denoted by fs but its value is not constant like kinetic friction force fk = ?kN static friction force is also called “self adjusting” static friction force value lies between 0≤ fs≤Flimiting  fs is static friction
minimum zero and Flimiting  = ?sN
don’t confuse there is no formula for static friction force because it is variable its value change hence it has a method not formula and ?sN is Flimiting maximum self adjusting force direction will be opposite to the tendency of relative motion where ?s is coefficient of static friction or coefficient of limiting friction see picture below.

from above question is clear that static friction force value is variable and self adjusting force hence if body is at rest then friction force will be equal to applied force which is justified body will be at rest not equal to the Flimiting force otherwise body will move opposite to applied force which is not possible logically this is important point to remember about static friction force now see the below question

from above question it is clear that body is in motion and friction acting upon body is kinetic friction so we can now find the acceleration using Newton’s law of motion Fnet = ma using this equation  20-8 = ma or 12 = 2a or a = 12/2 = 6m/s².

From the above question and FBD mg force try to slid the block on wall so first we will calculate Flimiting force = Static coefficient*Normal   here normal force will be equal to 40N because pushes the wall and wall will push with same force to block as normal force hence Flimiting = 0.3*40 = 12N now here applied force is mg force so mg = 1*10 = 10N now Flimiting is greater than mg force hence block will not slid so block will be at rest hence friction force will be 10N equal to mg force since block is at rest hence acceleration will be zero this is the concept for static friction force.
from the figure angle of friction is define as at the time of Flimiting the angle made between resultant contact force and normal reaction force is called angle of friction and it can be calculated as
tan?  = Flimiting/N  = ?sN/N = ?s , tan?= ?s this is angle of friction you can solve more question based upon the concept learn here now i hope you have enjoyed to learn Static friction force