Threshold for static friction

[aspodkfpo]

Homework Statement

Does the object begin to move at the threshold of motion?

Relevant Equations

none

Just wondering, what is the general consensus on the undifferentiable point in a graph of static friction and kinetic friction values regarding whether the object moves or doesn't move? Also wondering what the space between that point and where kinetic friction straightens out is called?

 

[haruspex]

Whether it moves or doesn’t right at that point has no practical consequence.
Why do you think it curves like that just to the right of that point? Could it drop vertically? Certainly, traversing right to left I doubt the friction will start to climb before motion ceases.

 

[aspodkfpo]

Whether it moves or doesn’t right at that point has no practical consequence.
Why do you think it curves like that just to the right of that point? Could it drop vertically? Certainly, traversing right to left I doubt the friction will start to climb before motion ceases.

Some questions say at what force is begins to move, so it does have use when solving questions and determining whether to put a > or = sign.

Google has multiple results of it not dropping vertically, and generally things tend to be smooth.

 

[haruspex]

Some questions say at what force is begins to move, so it does have use when solving questions and determining whether to put a > or = sign.

No, that does not follow. Does it begin to move when the force equals some particular value or when it exceeds it? There is no way to distinguish.

Regarding the transition, seems we need to allow that not all parts of the surface transition at the same instant. There will be some elasticity in the bodies. This gives the smooth curve. Not sure how different it wouid be with the reverse transition.

 

[aspodkfpo]

No, that does not follow. Does it begin to move when the force equals some particular value or when it exceeds it? There is no way to distinguish.

Actually, nevermind that.

So what about the curve, any name for it, is there a curve?

 

[haruspex]

So what about the curve, any name for it, is there a curve?

I had not come across this before, but I see how it could work.
Think of the block as made of jelly. As you pull on one end, say, it will distort, leading to a higher demand for frictional force at the leading end, so the static friction gives way there first. If the applied force is increased extremely slowly, it might even be possible to be in a state where parts of the surface, though static, have partly detached.

Or even if we hold the force constant after this detachment has started, maybe it will spread, so the shape of the curve depends on how slowly we increase the force.

Bottom line is that friction is really quite a complicated topic. The school version is very simplified.