# Frictional force

1. Jun 5, 2015

### cbram

Does the static friction get converted into kinetic friction while moving

2. Jun 5, 2015

### 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]

Last edited: Jun 5, 2015
3. Jun 5, 2015

### cbram

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

4. Jun 5, 2015

### jbriggs444

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.

5. Jun 5, 2015

See this

6. Jun 5, 2015

### cbram

Thank you very much

7. Jun 5, 2015

### 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.​