Question about Kinetic Friction

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Homework Help Overview

The discussion revolves around a problem involving kinetic friction and the coefficients of static and kinetic friction in the context of a block on an inclined plane at an angle of 28 degrees. The original poster is attempting to calculate the kinetic friction coefficient (mu_k) while considering the conditions for translation equilibrium.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster has calculated the normal and frictional forces to derive a value for mu_k, but questions the relationship between static and kinetic coefficients when an object is in equilibrium. Other participants clarify the differences between static and kinetic friction and suggest considering the forces acting on the block.

Discussion Status

Participants are engaged in clarifying concepts related to static and kinetic friction coefficients. Some guidance has been offered regarding the conditions under which these coefficients apply, and there is an ongoing exploration of how to approach the calculation of the kinetic coefficient.

Contextual Notes

The discussion includes assumptions about the conditions of motion and equilibrium, as well as references to specific values for the coefficients of friction from an external source. There is an implicit understanding that the original poster is working within the constraints of a homework assignment.

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Homework Statement



Run a trial with an angle of 28 degrees so the block is in translation equilibrium. Show how to calculate the mu_k value.
http://www.lon-capa.org/~mmp/applist/si/plane.htm

Homework Equations



F_fk = mu_k * F_n

The Attempt at a Solution



I have solved for Normal force and the frictional force and plugged them into the equation to get the coefficient of friction, I get roughly 0.53.
However, on the site given above, to get it in rough equilibrium, the mu_k is 0.44 while the mu_s is roughly 0.53

Just wondering, if an object is in equilibrium, are both coefficients the same?
 
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No. The coefficient of static friction will pretty much always be larger, the reason being that it's harder to get something moving than to keep it moving.

If it's in equilibrium and you're measuring the coefficient of friction by when it begins to move, that's the static coefficient. If it's moving and you're trying to measure when it's moving at a constant velocity, that's going to be the kinetic coefficient. Does that help?
 
Yes that does help, Thanks

But any ideas on how to solve for it?
 
Solve for which? Static or kinetic coefficient?
 
Kinetic
 
Okay, well think about it. The simplest way to find that is to set the frictional force equal to the force down the incline. What would have to be the case for you to be able to do that if the object is already moving?
 

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