Frictional Force: Static to Kinetic Conversion

[cbram]

Does the static friction get converted into kinetic friction while moving

 

[Simon Bridge]

No.

Friction is an emergent property of underlying intermolecular forces - it's an "effect" of something else.
So one kind of friction does not convert to another kind - better to think this way:

kinetic friction takes over from static friction at the instant the object starts moving.

For an applied force ##F##, with static and kinetic friction ##f_s## and ##f_k## respectively, we can find the acceleration as:
$$a = \begin{cases}
0 &: F<f_s\\
(F-f_k)/m &: F\geq f_s \end{cases}$$

[edited to tidy up the notation]

 

[cbram]

At the instance when f_k take over f_s does f_s becomes zero

 

[jbriggs444]

At the instance when f_k take over f_s does f_s becomes zero

I does not become zero so much as it becomes irrelevant. When there is relative motion, f_s does not apply -- the frictional force between the surfaces is given by f_k. When there is no relative motion, f_k does not apply -- the frictional force between the surfaces is limited by f_s.

 

[theodoros.mihos]

See this

 

[cbram]

Thank you very much

 

[Simon Bridge]

Note: if ##f## (no subscript) is the friction force, then Newtons law says ##F-f=ma## where

##f=\begin{cases} f_k &: F > f_s\\ F &: F < f_s \end{cases}##

What happens ##F=f_s## technically depends on the wording of the question - which is why I'm being deliberately ambiguous about where the "equality" goes.​