I can not assume the sense of the friction force?

AI Thread Summary
In statics problems, the direction of mechanical forces can be assumed, with negative results indicating incorrect assumptions. However, the friction force cannot be assumed independently, as its direction is dependent on the motion of the object; for instance, if an object is assumed to slide right, friction must act left. The equation F = uN does not serve as a vector equation, limiting its use for determining direction. This is due to the perpendicular relationship between normal and frictional forces in typical scenarios. Understanding these principles is crucial for accurately analyzing friction in mechanics.
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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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