Friction

Friction is generally defined as the "force that opposes" motion.

When the motion of an object is analyzed, one must take into consideration that
There are two types friction forces THAT MAY BE acting on the object.

i) Before the object starts to move     - Static friction

ii) While the object is moving             -Kinetic friction

In our laboratory experiments we also found that friction depends on several factors:

1. The weight of the object i.e. the force of gravity ( F_g)
    
   recall that        F_g = m x g
    

2. Therefore,  indirectly friction depends on mass 
    

3. Friction also depends on the type of surface

i.e. Friction was higher on rough surfaces (sand paper vs. plastic)

Technicians have classified surfaces using a degree of roughness called  the Coefficient of Friction 

The higher the Coefficient of friction, the rougher the surface .: the higher  the friction force.

For our purposes we will use the equation

Note: The force of friction dose not depend on surface Area.

Example:
 A 200 Kg crate of bricks ( made of wood) is pulled along a rubber floor. The coefficient of friction is 0.38. How much force is needed to move the crate?

given: m = 200 Kg
                       g = 9.8 m / s2
                        u = 0.35

Find: F_f

Solution:        F_f =  µ m g  

                            = ( 0.35) (200 Kg) ( 9.8 m / s2)

                            = 686 Kg m (s2)  =  686 (N)

.: The minimum force required to get the crate moving is 686N