Cone of Friction: When the body is having impending motion
in direction of P, the frictional force will be limiting friction and the resultant reaction R will make limiting friction angle θ with the normal. If body is having impending motion in another direction, again the resultant reaction makes limiting frictional angle with normal in that direction. Hence, when the direction of force P is changed through 360° gradually, the resultant R generates a right circular cone with the semi-central angle equal to θ .
If the resultant reaction is on surface of this inverted right circular cone the semi central angle of which is limiting frictional angle (θ), the motion of body is impending. If the resultant is within this cone, body is stationary. This inverted cone having semi central angle, which is equal to the limiting frictional angle θ, is called as cone of friction.
It can be defined as the right circular cone having vertex at the point of contact of two bodies (or the surfaces), axis in direction of normal reaction (R) and semi vertical angle equal to the angle of friction (θ). Figure which is at the adjacent shows the cone of friction in which,
O = Point of contact between the two bodies.
R = Normal reaction and also axis of cone of friction.
θ= Angle of friction