Understanding the Relationship Between Static and Kinetic Friction

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Discussion Overview

The discussion centers around the relationship between static and kinetic friction, exploring why static friction is generally greater than kinetic friction. Participants delve into the underlying mechanisms, definitions, and implications of friction in various contexts, including theoretical and practical considerations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest that the force required to initiate movement (static friction) is greater than that needed to maintain movement (kinetic friction), attributing this to the roughness of surfaces and the time for surface irregularities to settle.
  • Others propose that friction is area-independent, emphasizing that the total normal force is what determines the frictional force, regardless of contact area.
  • A participant mentions that friction may have an electromagnetic origin, questioning how this relates to the observed differences between static and kinetic friction.
  • Some contributions highlight the mathematical definitions of static and kinetic friction, discussing how kinetic friction is derived from static friction and the normal force.
  • There are challenges to the explanations provided, with participants seeking clarification on concepts like the role of contact area and the nature of adhesive forces at the atomic level.
  • One participant raises a hypothetical scenario to illustrate a contradiction if kinetic friction were to exceed static friction, further complicating the discussion.

Areas of Agreement / Disagreement

Participants express a range of views, with no consensus reached on the reasons behind the differences between static and kinetic friction. Multiple competing explanations and models are presented, indicating ongoing debate and exploration of the topic.

Contextual Notes

Some discussions reference the simplifications made in models of friction, such as the assumption of smooth surfaces, and the breakdown of these models under certain conditions, like highly deformable surfaces.

Who May Find This Useful

This discussion may be of interest to students and professionals in physics, engineering, and materials science, particularly those exploring the fundamentals of friction and its applications.

shanu_bhaiya
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Does anyone know what is the reason behind that static friction is more than
kinetic friction?
Many people give reasons but they forget that friction is area independent force.
Please help me...:confused:
 
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shanu_bhaiya said:
Does anyone know what is the reason behind that static friction is more than
kinetic friction?
Many people give reasons but they forget that friction is area independent force.
Please help me...:confused:

Because the amount of force required to move an object starting from rest is usually greater than the force required to keep it moving at constant velocity once it is started.

regards
marlon brando
 
shanu_bhaiya said:
Does anyone know what is the reason behind that static friction is more than
kinetic friction?
Many people give reasons but they forget that friction is area independent force.
Please help me...:confused:

In reality it's probably some complicated solid state physics boundary problem, but I'll tell you how I think of it ;)

Friction basically arises because the edges of the two surfaces in contact are rough. If the surfaces aren't in motion relative to each other and they've had a bit of time to settle the protruding bits of the one surface have nestled into the valleys of the other, they're stuck together pretty well. This gives you your coefficient of static friction. If the surfaces are moving relative to each other all the edges are hopping along, skipping along the surface. They don't have enough time to settle down and really get a good grip, this gives rise to the coefficient of kinetic friction.
 
dicerandom said:
In reality it's probably some complicated solid state physics boundary problem, but I'll tell you how I think of it ;)
Friction basically arises because the edges of the two surfaces in contact are rough. If the surfaces aren't in motion relative to each other and they've had a bit of time to settle the protruding bits of the one surface have nestled into the valleys of the other, they're stuck together pretty well. This gives you your coefficient of static friction. If the surfaces are moving relative to each other all the edges are hopping along, skipping along the surface. They don't have enough time to settle down and really get a good grip, this gives rise to the coefficient of kinetic friction.

Let's suppose two bodies with different contact surface areas of same mass.

Now, the friction acting on both the bodies will be same if the surface is same.

According to your assumption, friction must be more in the case in which contact surface area will be more, because each part of the body's surface will possesses its own frictional repose. And each part's repose will be added to give the total friction.

If I've misunderstood your explanation, please explain more briefly, with diagrams if possible...

and, I've also listened that FRICTION HAS ELECTROMAGNETIC ORIGIN, can this be the reason behind that, if yes please, explain...:rolleyes:
 
shanu_bhaiya said:
According to your assumption, friction must be more in the case in which contact surface area will be more, because each part of the body's surface will possesses its own frictional repose. And each part's repose will be added to give the total friction.
While it's true that each part of the contact area adds to the total friction, what also matters is the pressure between the contact surfaces: It turns out that it's just the total normal force between the two surfaces that counts (in a simplified model of friction that applies well enough to cases where the surfaces are smooth and hard enough). So each object, since they weigh the same, would experience the same friction force.

(This simple model breaks down when the surfaces are highly deformable, so that they can better fill the nooks and crannies between them. That's one reason why drag racers have wide tires.)

and, I've also listened that FRICTION HAS ELECTROMAGNETIC ORIGIN, can this be the reason behind that, if yes please, explain.
All contact forces between objects are electromagnetic in origin. Meaning that the fundamental force involved is the electromagnetic interaction between the charges in each surface (as opposed to being nuclear or gravitational force).
 
Bhaita jara khud bhi socho.
I don't find hisexplanation of settling down as something that does not explain the reason when friction is considered due to charge. When a body is moving there is gain a constantbreaking of attraction between the two surfaces.
 
Doc Al said:
While it's true that each part of the contact area adds to the total friction, what also matters is the pressure between the contact surfaces: It turns out that it's just the total normal force between the two surfaces that counts (in a simplified model of friction that applies well enough to cases where the surfaces are smooth and hard enough). So each object, since they weigh the same, would experience the same friction force.
(This simple model breaks down when the surfaces are highly deformable, so that they can better fill the nooks and crannies between them. That's one reason why drag racers have wide tires.)
All contact forces between objects are electromagnetic in origin. Meaning that the fundamental force involved is the electromagnetic interaction between the charges in each surface (as opposed to being nuclear or gravitational force).

I hardly understood what you want to say, I live in India and my English is very weak...
 
Actually he want to say

1. limiting friction is proportional to N/A

and
2. limiting friction is proportional to A

hence

limiting friction is proportional N

that is why we generally say that the limiting friction is in dependent of the area in contect.

MP
 
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Friction is ue to the adhesive force between atoms in the tiny joints. Due to this force two different particles tend to stick together.

and as stated above friction is independent of area but dependent on the normal reaction that is generally weight of a body.

-Benzun
 
  • #10
shanu_bhaiya said:
Does anyone know what is the reason behind that static friction is more than kinetic friction?
It seems to me it is built into the way kinetic friction is defined. Kinetic friction is the static friction divided by the normal force. Since you're always starting with the static friction and dividing it by some other figure, you'll never end up with a result that's larger than what you divided.
 
  • #11
That is not a bad thought but you are forgetting that if the Normal Force is less than 1, and if static friction is indeed divided by normal force to obtain kinetic friction, then the kinetic friction could, by definition, be greater than static friction.

By the way, isn't the static friction between two objects equal to the normal force X the coefficient of static friction. This would mean that dividing the static friction by the normal force would give you the coefficient of static friction which is definitely not equal to the kinetic friction between the two objects.
 
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  • #12
I wrote this in another thread, but repeat here :rolleyes:

If kinetic friction is greater than static friction, you'll have a contradiction. (but this won't explain they can't be the same.)

Assume kinetic friction is greater than static friction. And you are pushing a box on a floor, gradually increasing the force. At a point the box begins to move when the force becomes greater than the static friction. However, it's still smaller than kinetic friction under our assumption, so it must not be moving. This is clearly a contradiction.
 
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  • #13
150grainhunter said:
By the way, isn't the static friction between two objects equal to the normal force X the coefficient of static friction. This would mean that dividing the static friction by the normal force would give you the coefficient of static friction which is definitely not equal to the kinetic friction between the two objects.
You're right. What I explained above is actually the formula for the coefficient of kinetic friction not the one for kinetic friction.

To get the Force of kinetic friction we have to take the above coefficient and multiply it by the normal force.
 

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