Friction

When sliding an abject on a surface, frictional forces need to be overcome. What is exactly friction? When an object is placed on a surface, electromagnetic forces are being formed between the object and the surface. These atomic bonds need to be detached for every instant during motion. Therefore, friction always acts in the opposite direction of the applied force and is dependent on two quantities. First, the magnitude of friction depends on the weight of the object. The heavier the object the stronger the frictional force. Second, the magnitude of friction depends on the properties of the surface. It is easier to slide ice on ice than on a piece of rug.
The magnitude of friction is expressed as

f = μN

where f is the frictional force, μ is the coefficient of friction, and N is the weight of the object. Since the frictional force is different for bodies at rest and for bodies at motion, there are two different denotations for μ.

1. μs is called coefficient of static friction and is used for bodies with an initial velocity of zero.

2. μk is called coefficient of kinetic friction and is used for bodies with an initial velocity of greater than zero. Since the body is already moving, it easier to overcome the frictional force.

Typical values for μs are the following:

Metal on metal: μs = 0.15-0.2
Masonry on masonry: μs = 0.60-0.70
Wood on wood: μs = 0.25-0.5
Metal on masonry: μs = 0.30-0.70
Metal on wood: μs = 0.20-0.60
Rubber on concrete: μs = 0.50-0.90

The higher the value for μs, the greater is the frictional force and thus, the force to overcome friction.