Friction is a resisting force which opposes the movement or relative motion of two surfaces in contact with each other. Friction does not let two bodies slide easily.
There are several types of friction
Friction between each surfaces can be calculated using an equation or idea given by Charles-Augustin de Coulomb. The equation is
.
In the simple case of a mass resting on a horizontal surface, the only component of the normal force is the force due to gravity, where 
. In this case, the magnitude of the friction force is the product of the mass of the object, the acceleration due to gravity, and the coefficient of friction. However, the coefficient of friction is not a function of mass or volume; it depends only on the material. For instance, a large aluminum block has the same coefficient of friction as a small aluminum block. However, the magnitude of the friction force itself depends on the normal force, and hence the mass of the block.
If an object is on a level surface and the force tending to cause it to slide is horizontal, the normal force 
between the object and the surface is just its weight, which is equal to its mass multiplied by the acceleration due to earth's gravity, g. If the object is on a tilted surface such as an inclined plane, the normal force is less, because less of the force of gravity is perpendicular to the face of the plane. Therefore, the normal force, and ultimately the frictional force, is determined using vector analysis, usually via a free body diagram. Depending on the situation, the calculation of the normal force may include forces other than gravity.
There are several types of friction
- Dry Friction: Friction that exists between two solid surfaces in contact.
- Fluid friction or viscosity: Friction that exists between layers of a viscous fluid.
- Skin friction: A force which opposes the movement of a solid through a fluid.
- Internal Friction: Friction that exists in a solid which opposes its deformation.
The Force of Friction exists when a body is stationary as well as moving. The friction that exist when a body is stationary is called static friction.
Static Friction makes a body stay in its position even though a small amount of external force is being applied(depends on the roughness of the surface). It is denoted by μs.
The static friction force must be overcome by an applied force before an object can move. The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of static friction and the normal force: 
. When there is no sliding occurring, the friction force can have any value from zero up to 
. Any force smaller than attempting to slide one surface over the other is opposed by a frictional force of equal magnitude and opposite direction. Any force larger than overcomes the force of static friction and causes sliding to occur. The instant sliding occurs, static friction is no longer applicable—the friction between the two surfaces is then called kinetic friction.
. When there is no sliding occurring, the friction force can have any value from zero up to . Any force smaller than attempting to slide one surface over the other is opposed by a frictional force of equal magnitude and opposite direction. Any force larger than overcomes the force of static friction and causes sliding to occur. The instant sliding occurs, static friction is no longer applicable—the friction between the two surfaces is then called kinetic friction.
Friction between a body and a surface when the body is moving is called Kinetic friction. It is denoted by μk.
The Kinetic Friction is smaller than Static Friction.
Angle of Friction: For certain applications it is more useful to define static friction in terms of the maximum angle before which one of the items will begin sliding. This is called the angle of friction or friction angle. It is defined as:
where θ is the angle from vertical and µ is the static coefficient of friction between the objects. This formula can also be used to calculate µ from empirical measurements of the friction angle.
Static Friction as limiting friction:
Static Friction behaves rather differently in different situations. Experiments show that static friction increases with the increase in external forces being applied. The friction keeps increasing until it meets a certain value or reaches its limit. Then the friction is no longer static. Its kinetic as told above. Thus we can conclude that its is harder to begin the motion of an object rather than keeping it in motion.
Calculating Force of Friction:
Friction between each surfaces can be calculated using an equation or idea given by Charles-Augustin de Coulomb. The equation is
.
where
is the force of friction exerted by each surface on the other. It is parallel to the surface, in a direction opposite to the net applied force.
is the coefficient of friction, which is an empirical property of the contacting materials,
is the normal force exerted by each surface on the other, directed perpendicular (normal) to the surface.
Concept of Normal Force: The normal force is defined as the net force compressing two parallel surfaces together; and its direction is perpendicular to the surfaces. From Newton's third law of motion, we can say that Normal force is a reaction to the force an object is applying on the surface.
In the simple case of a mass resting on a horizontal surface, the only component of the normal force is the force due to gravity, where . In this case, the magnitude of the friction force is the product of the mass of the object, the acceleration due to gravity, and the coefficient of friction. However, the coefficient of friction is not a function of mass or volume; it depends only on the material. For instance, a large aluminum block has the same coefficient of friction as a small aluminum block. However, the magnitude of the friction force itself depends on the normal force, and hence the mass of the block.
If an object is on a level surface and the force tending to cause it to slide is horizontal, the normal force between the object and the surface is just its weight, which is equal to its mass multiplied by the acceleration due to earth's gravity, g. If the object is on a tilted surface such as an inclined plane, the normal force is less, because less of the force of gravity is perpendicular to the face of the plane. Therefore, the normal force, and ultimately the frictional force, is determined using vector analysis, usually via a free body diagram. Depending on the situation, the calculation of the normal force may include forces other than gravity.
Laws of Friction:
The elementary properties of sliding (kinetic) friction were discovered by experiment in the 15th to 18th centuries and were expressed as three empirical laws:
- Amontons' First Law: The force of friction is directly proportional to the applied load.
- Amontons' Second Law: The force of friction is independent of the apparent area of contact.
- Coulomb's Law of Friction: Kinetic friction is independent of the sliding velocity.
Amontons' 2nd Law is an idealization assuming perfectly rigid and inelastic materials. For example, wider tires on cars provide more traction than narrow tires for a given vehicle mass because of surface deformation of the tire.
Fluid Friction: Fluid friction occurs between layers within a fluid that are moving relative to each other. This internal resistance to flow is described by viscosity. In everyday terms viscosity is "thickness". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. Put simply, the less viscous the fluid is, the greater its ease of movement.
All real fluids (except superfluids) have some resistance to stress and therefore are viscous, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid.
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