Coefficient of friction and static friction

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Discussion Overview

The discussion revolves around the relationship between the coefficients of static and dynamic friction when an object slides down a slope at a constant speed. Participants explore whether these coefficients are the same in all situations and the implications of their differences in various contexts.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question whether the coefficient of static friction is the same as the coefficient of dynamic friction when an object is sliding down a slope at constant speed.
  • One participant clarifies that static friction applies when two bodies are stationary relative to each other, while kinetic friction applies when they are in relative motion, suggesting a fundamental difference between the two types of friction.
  • Another participant presents calculations for a box sliding down a slope at a 45-degree angle, arguing that the coefficients appear to be the same under certain conditions, but acknowledges that they are typically different.
  • Some participants assert that the relationship between frictional force and normal force is different for static and dynamic friction, with static friction representing a maximum value.
  • One participant discusses the conditions under which the coefficients might be equal, suggesting that this occurs when the slope angle and coefficient of dynamic friction are balanced, leading to no acceleration.
  • There is mention of specific surface pairs, such as teflon on teflon, where static and dynamic friction coefficients may be similar.

Areas of Agreement / Disagreement

Participants express differing views on whether the coefficients of static and dynamic friction are the same in all situations, with some asserting they are generally different while others present scenarios where they might appear equal.

Contextual Notes

Participants highlight the importance of conditions such as slope angle and surface interactions, which may affect the coefficients of friction. There is also an acknowledgment of the limitations in the calculations presented, particularly regarding the assumptions made about the forces involved.

p.tryon
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If an object slides down a slope at a constant speed is the coefficient of static friction the same as the coefficient of dynamic friction? If yes, is this true in every situation?
 
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p.tryon said:
If an object slides down a slope at a constant speed is the coefficient of static friction the same as the coefficient of dynamic friction? If yes, is this true in every situation?
I'm afriad that I don't understand the question. Static friction occurs when two bodies are stationary relative to each other, whereas kinetic friction occurs when the two bodies are in relative motion.

When two bodies moving relative to each other are in contact, there is said to be no static friction.
 
Imagine a box of weight W sliding at a constant speed down a slope at an angle of 45 degrees. Imagine the same box at rest on the same slope. The two types of friction are different (static and kinetic) and the two coefficients of friction should be different (or should they?)

According to the following calculations they are the same

From here on:

Coefficient of friction C
Frictional force F
Normal Force Fn

F = C.Fn
Therefore
C = F / Fn

1. At rest:

F = WSin45 and Fn = Wcos45
Therefore

C = WSin45 / Wcos45

2. Sliding at a constant speed:

Since F is equal to the opposite component of gravity (WSin45) - Newton's first law! the equation and therefore answer is exactly the same for the coefficient of dynamic friction:

C = WSIn45 / Wcos45
 
p.tryon said:
Imagine a box of weight W sliding at a constant speed down a slope at an angle of 45 degrees. Imagine the same box at rest on the same slope. The two types of friction are different (static and kinetic) and the two coefficients of friction should be different (or should they?)
In general, they would be different.

According to the following calculations they are the same

From here on:

Coefficient of friction C
Frictional force F
Normal Force Fn

F = C.Fn
Therefore
C = F / Fn
The relationship F = μN is only true for kinetic friction.

For static friction, μN represents the maximum value, so F ≤ μN. Big difference!
 
p.tryon said:
If an object slides down a slope at a constant speed is the coefficient of static friction the same as the coefficient of dynamic friction?
This would only imply that the slope angle and coefficient of dynamic friction were "balanced" such that they resulted in equal and opposing forces from gravity and dynamic friction (therefore no acceleration), and that dynamic friction was independent of speed within the speed range experienced, excluding a speed of zero. The constant speed (not including zero speed) case occurs when tan(slope angle) = coefficient of dynamic friction.

If the speed were zero, then the friction would be due to a normally higher coeffcient of static (than dynamic) friction, and the slope could be increased without the box moving. If the box was pushed, then the box would accelerate down the steeper slope instead of sliding at constant speed.

There are pairs of surfaces where static and dynamic friction are about the same, such as teflon on teflon.
 
Last edited:

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