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Frictional Forces

By Edited Nov 13, 2013 0 0

Frictional forces are those that resist the movement of a body in motion.  These forces are encountered when the body is traveling over a rough surface as sliding friction or through a highly viscous environment.


The term viscous, typically referred to in the context of a fluid property is the level of friction inherit in a medium such as oil or atmospheric air.  The higher the level of viscosity in the medium the greater the level of friction force resisting the body while traveling through that medium.


Frictional forces are something that can both benefit and make our lives tougher.  If for example, you are pushing a heavy trunk across a rough concrete driveway, the interface between the trunk and driveway will create a resisting frictional force.  This situation will require you to apply more force by pushing harder to overcome the friction.  However, if not for the roughness of the concrete creating this friction, you would have no traction to push the trunk or even walk for that matter.  After an ice storm in winter when this driveway is covered in ice and quite slippery, you would certainly be wishing for some friction especially when you have to walk across it to get to your car.


Friction that occurs as an object is moving across a surface or sliding friction as it is called occurs in two forms, static and kinetic.


The force of static friction is the force that arises as you are moving an object from rest along a surface.  The more force you apply to the object, the greater the developed resisting force will be up until the point where the force you apply overcomes the resistance and the object begins to move.  The force you apply up to the point of the object moving is equal to the maximum static friction force, which can be found from the relationship, Fr,s = MusN.  Fr,s is the static friction force, Mus is the coefficient of static friction dependent on the interface of the object and the surface, where N is the normal force acting upward on the object which is equal to the weight of the object.


The force of kinetic friction is the resisting force an object encounters while in motion and is therefore equivalent to the minimum force required to keep the object in motion.  It should be noted that the force of kinetic friction will be by varying degrees less than the force of static friction.  The minimum kinetic friction force can be found from the relationship, Fr,k = MukN.  In this equation, Fr,k is the kinetic friction force, Muk is the coefficient of kinetic friction, and N is the normal force equal to the weight of the object being moved.


When an object is traveling through a highly viscous medium, there are two main categories to consider. 


In the first category, we consider extremely small and slowly moving objects free falling through a viscous medium such as air or oil and assume that the objects velocity is proportional to the developed resisting force.  This resisting force acting on the object can be determined from the relationship, RF = cv, where RF is the resisting force, c is a constant material property of the medium, and v is the objects velocity.


In the second category, we consider moving objects with high speeds such as a car and large objects free falling in some medium and assume that the square of the objects velocity is proportional to the developed resisting force.  This resisting force acting on the object can be determined from the relationship, RF = ½ CpAv2, where RF is the resisting force, C is the drag coefficient, p is the density of the medium, A is the objects cross sectional area projected in the direction of motion, and v is the objects velocity.




Serway, R. A., Beichner, R. J., & Jewett, J. W. (2000).  Physics For Scientists and      Engineers, Volume 1 (5th Ed.).  Orlando, FL: Saunders College Publishing.



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