Friction Information



Friction is a force which is resistant to a body’s motion due to interactions with its surroundings.  Friction can be broken into two main parts:  Static Friction and Kinetic Friction.


Static Friction

Static Friction is used when an object is not moving.  Static frictional forces due to interaction between the surfaces will increase in proportion to the applied force up until some limit occurs, and motion of the object begins.  This limit is commonly referred to as the threshold of motion.  One can find the static friction using the following equation:


                Ff  <  ms  *  F                        Ff = Friction force

                                                                                 ms = coefficient of static friction

                                                                                 Fn = Normal Force


The reason why it a “less than or equal to” sign is that for when the applied force is before the threshold of motion, we use and equals to sign.  When we pass the threshold, the frictional force becomes less than the applied force, meaning the forces are unbalanced and there will be motion.



Kinetic Friction

Kinetic Friction is used when an object is moving.  Unlike static friction, kinetic friction is a constant once the object starts moving.  It can be found using the equation:


        Ff  =  mk  *  Fn                           Ff = Friction force

                                                                             mk = coefficient of kinetic friction

                                                                              Fn = Normal Force


By looking at the equations for static and kinetic coefficients of friction, one can generate the following friction assumptions:


Good Approximations When Dealing with Friction


1)         Frictional force is directly proportional to load, that is, the total force, which acts normal to the sliding         surface. 

2)         Frictional force for a constant load is independent of the area of contact.


3)         Frictional force is independent of the velocity of sliding.


4)         Frictional force depends upon the nature of materials in contact





How does this relate to skiing?

     When you use wax on your skis, you are altering the friction force by changing the nature of materials in contact (approximation #4).  Without any wax, there isn't much friction between your skis and the snow, and it is very difficult to grip the snow (similar to walking on ice).  The same situation is seen if you use too hard of a wax for the snow conditions.  However, if you use too soft of a wax, there will be too much friction, an you won't be able to slide on the snow at all.  This is why choosing the right wax for the snow conditions is very important for cross-country skiers.




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