# Highlights

(a)        Friction

1.   Force of friction always acts in the direction opposite to the direction of motion.
2.   The maximum value of frictional force acting on a body, when the body is on the point of motion, is called limiting force of friction. It is denoted by F.
3.   The force of friction, acting on a body when the body is moving, is called dynamic friction.
4.   The ratio of the limiting force of friction (F) to the normal reaction (R) between two bodies is known as co-efficient of friction. It is denoted by µ. Mathematically, µ=F/R.
5.   The angle made by the resultant of the normal reaction (R) and the limiting force of friction (F) with the normal reaction is known as angle of friction. It is denoted by Φ.
6.   The relation between angle of friction (Φ) and co-efficient of friction (µ) is expressed as tan Φ = µ.
7.   The force of friction is always equal to µR, where R is normal reaction.
8.   If a ladder is leaning against a smooth vertical wall, the force of friction between ladder and vertical wall will be zero.
9.   The problems on ladder friction are solved by taking algebraic sum of horizontal and vertical components of the forces to be zero and also taking the moments of all forces about any point to be zero.
10.  If a body is placed on a rough inclined plane and the angle of inclination of the plane is gradually increased, till the body just starts sliding down the plane. The angle of the inclined plane, at which the body just begins to side down the plane, is called angle of repose.
11.   Angle of repose is equal to angle of friction.
12.  If the inclination of the plane, with the horizontal is less than angle of friction, the body placed on the inclined plane will be always in equilibrium without any external force.
13.  The minimum force required to drag a body of weight W placed on a rough horizontal plane, will be equal to angle of friction for the least force.
14. The force required to drag a body of weight W, placed on a rough inclined plane of inclination α to the horizontal, when force is applied to the body in such a way that the force makes an angle α with the inclined plane is given by

(B) Belt Friction