odies in contact. It is always opposite to the direction of motion. Cause of Friction The cause of friction are:- I)The microscope irregularities between two surfaces in contact. II)The intermolecular force of attraction. The magnitude of force of friction depends on :- A)The nature of two surfaces in contact. B)Actual area in contact between them. C)The normal reaction force between the two surfaces. Types of Friction Friction is of following three types :- Sliding Friction Rolling Friction Fluid Friction Sliding Friction The force of friction which comes into act when one body slides over the other is called as sliding friction. Sliding friction is weaker than static friction. That’s why it’s easier to slide a piece of furniture over the floor after you start it moving than it is to get it moving in the first place. Sliding friction can be useful. For example, you use sliding friction when you write with a pencil. The pencil “lead” slides easily over the paper, but there’s just enough friction between the pencil and paper to leave a mark. Sliding friction is also of following types. :- A)Static B)Limiting C)Dynamic A)Static Friction It is the force of friction which comes into act when one body tends to slide over the other but actually there is no motion. B)Limiting Friction It is the maximum value of force of friction so long as there is no actual motion between two bodies in contact, i.e. maximum value of static friction. C)Dynamic or Kinetic Friction It is the force of friction which comes into act when one body actually moves over the other. It is independant of the velocity of the body. Rolling Friction The force of friction which comes into act between two bodies when one body rolls over the other is called as rolling friction. Force of rolling friction is less than that of sliding friction as the actual area in contact is less in that of rolling friction. Rolling friction is much weaker than sliding friction or static friction. This explains why most forms of ground transportation use wheels, including bicycles, cars, 4-wheelers, roller skates, scooters, and skateboards. Ball bearings are another use of rolling friction. Fluid Friction The force of friction which comes into act when one body moves in a fluid is called as fluid friction. A fluid is a substance that can flow and take the shape of its container. Fluids include liquids and gases. If you’ve ever tried to push your open hand through the water in a tub or pool, then you’ve experienced fluid friction. Angle of Repose Angle of repose is the angle made by the inclined plane with the horizontal such that the body placed over it begins to slide. Angle of friction Angle of friction is the angle made by the resultant of normal reaction force and force of friction with the normal reaction force To see the formulas of the above definitions, refer to the pdf given