Difference between coefficient of static and kinet friction

[bookerdewitt]

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?

 

[A.T.]

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.

 

[bookerdewitt]

So mg=ffs would only hold if the block is just starting to move?

 

[cepheid]

So mg=ffs would only hold if the block is just starting to move?

Yeah, dude. The expression for static friction is F_s ≤ μ_sN, where N is the normal force, and μ_s is the coeff. of static friction. So, μ_sN 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.

 

[dauto]

No. mg=ffs would hold but ffs = mu * N wouldn't.

 

[bookerdewitt]

Ok. That makes sense.