Difference between coefficient of static and kinet friction

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The discussion clarifies the difference between static and kinetic friction coefficients in a pulley system scenario. It explains that the force of static friction can vary from zero up to a maximum value defined by the coefficient of static friction, μs, multiplied by the normal force. The equation mg = ffs only applies when the block is on the verge of moving, not when it is stationary. Once the block begins to move, the force of kinetic friction, represented by μk, comes into play, which is typically lower than μs. This distinction highlights the conditions under which each coefficient applies and the nature of frictional forces.
bookerdewitt
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Lets say you have a block on a horizontal surface connected to a pulley and then a weight hanging over the edge. If its not moving then the force applied which is just the mass of the weight x gravity is equal to the force of static friction μsmblockg. So that means μs = the mass of the weight/the mass of the block. But if the block is moving at a constant velocity wouldn't the force applied also equal the force of kinetic friction which would mean the μk also equals the ratio of the masses? I thought the coefficient of static friction is always higher than the coefficient of kinetic friction? Where am I going wrong?
 
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bookerdewitt said:
If its not moving then the force applied which is just the mass of the weight x gravity is equal to the force of static friction μsmblockg.
No. The static friction coefficient tells you the maximal static friction force, for a given normal force. The actual static friction force can be anything between 0 and that value.
 
So mg=ffs would only hold if the block is just starting to move?
 
bookerdewitt said:
So mg=ffs would only hold if the block is just starting to move?

Yeah, dude. The expression for static friction is Fs ≤ μsN, where N is the normal force, and μs is the coeff. of static friction. So, μsN is the maximum static friction that can be provided. Anytime before the maximum static friction is overcome and the object starts moving, the static friction force is less than that, because it is only whatever value it needs to be to prevent motion, given the other applied forces on the body.
 
No. mg=ffs would hold but ffs = mu * N wouldn't.
 
Ok. That makes sense.
 

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