I can not assume the sense of the friction force?

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SUMMARY

The discussion centers on the challenges of determining the direction of friction force in statics problems, particularly when using the equation F = uN. Unlike other mechanical forces, the direction of friction is dependent on the motion of the object; if an object is assumed to slide to the right, the friction force must act to the left. The participants emphasize that the equation F = uN is not a vector equation, as the normal force and frictional force are typically perpendicular, complicating the assumption of direction.

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  • Understanding of static equilibrium principles
  • Familiarity with the equation of friction F = uN
  • Knowledge of vector forces and their components
  • Basic mechanics concepts, including normal force
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  • Explore vector analysis in physics, focusing on force components
  • Learn about the implications of friction in different motion scenarios
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With all the mechanical forces in statics problems I can assume the sense and I might get a negative answer which implies I made the wrong assumption. But why can I not do this with the friction force when using the equation F=uN?
 
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Because the direction of friction is not independent of other variables. For example, if you assume something slides to the right, then friction on it must point left.

Perhaps discussing a specific example would help, if you have one.
 
Besides, the equation you mention is not a vector equation, so you can't use it to find direction. It can't be a vector equation, because in ordinary cases the normal force and frictional force are perpendicular.
 

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