# Frictional Force: Static to Kinetic Conversion

• cbram
In summary, friction is an emergent property of underlying intermolecular forces and does not convert from static to kinetic. Kinetic friction takes over from static friction when an object starts moving and the acceleration can be determined using the applied force and the two types of friction. At the point when kinetic friction takes over, static friction becomes irrelevant. The frictional force is limited by static friction when there is no relative motion and by kinetic friction when there is relative motion. The friction force can be calculated using Newton's law, where the force is equal to the applied force minus the friction force, which is either kinetic or equal to the applied force. When the applied force is equal to the static friction force, the outcome depends on the specific question being
cbram
Does the static friction get converted into kinetic friction while moving

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

cbram said:
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.

Thank you very much

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.​

## 1. What is frictional force?

Frictional force is a type of force that opposes the motion of an object when it is in contact with another surface.

## 2. How is frictional force related to static and kinetic friction?

Static friction is the force that prevents an object from moving when it is at rest, while kinetic friction is the force that opposes the motion of an object that is already in motion. Frictional force is responsible for both static and kinetic friction.

## 3. How does frictional force convert from static to kinetic?

Frictional force converts from static to kinetic when the force applied to an object is greater than the force of static friction. This causes the object to start moving, and the frictional force changes to kinetic friction to oppose the motion.

## 4. What factors affect the conversion from static to kinetic friction?

The conversion from static to kinetic friction can be affected by factors such as the type of surface the object is on, the weight of the object, and the force applied to the object. It can also be affected by the presence of lubricants or the temperature of the surfaces.

## 5. How is frictional force important in everyday life?

Frictional force plays a crucial role in our daily lives, from walking on the ground to driving a car. It allows us to grip objects, prevents us from slipping, and helps us control the speed of our movements. It also allows machines to function properly and helps us to slow down or stop moving objects.

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