Angle of friction is the angle between the normal reaction and resultant of normal reaction and frictional force.When a body is on a horizontal surface,its weight always acts in vertically downward direction.When we apply a horizontal force to move the body, frictional force starts acting in opposite to the direction of applied force.
Normal reaction is the reaction applied by lower body on the upper body against the force acting on the lower body and it shall be normal to the surface of contact.
We can identify that normal reaction and frictional force are perpendicular to each other.Their resultant will be in between them and its value can be determined using parallelogram law of vectors.This resultant makes some angle with normal reaction and this angle is called angle of friction.
It is mathematically proved below that angle of contact depends on the nature of the surface and its roughness.Hence it depends on coefficient of friction of the surface.
We shall apply enough force to move the body against the maximum static frictional force and if the applied force is less than that, body remains in the state of rest.Here static frictional force self adjusts itself to the applied force and the body remains in the state of rest.This is called self adjusting nature of static frictional force.
If the applied force is greater than limiting frictional force,then the body starts moving in the direction of applied force.We can calculate the resultant force as applied force subtracting the frictional force as it is acting against the motion.We can also calculate the resultant acceleration that the body is going to get.
In the following video all this is explained in detail.
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