How to calculate mew S and mew K?

[hamid14]

I have average Fs and Fk which is static force and kinetic force and i need to calculate mewS which is a wierd u and then s and i also need to calculate mewK.

The question says to calculate for coefficient of friction

[TerranceN]

First of all it is spelled mu not mew (it confused me at first, lol)

Static coefficient of friction is what is used to find the amount of force it takes to make something move from rest, and kinetic coefficient of friction is what is used to find the frictional force when the object is already in motion.

in general Ff = μ * Fn (where Ff is friction force and Fn is normal force)

therefore

Fs = μs * Fn
and
Fk = μk * Fn

Rearranged:
μs = Fs/Fn
and
μk = Fk/Fn

[hamid14]

so if i have Fn is equal to Fg i can use 9.81m/s[down] for Fn?

[jcollins1991]

hamid14 @ Tue Nov 02, 2010 4:35 pm wrote:

so if i have Fn is equal to Fg i can use 9.81m/s[down] for Fn?

9.8 is the acceleration, you need to multiply by the weight to get the force (f = ma)

[hamid14]

the weight us 1kg, would i need to convert o grams? or can i keep it as kg?

so,

Fn = ma

if m = 1kg, so then a = -9.81 or +9.81?

Fn = 1kg * -9.81?

[TerranceN]

Kg is the standard unit for mass.

You get to choose what directions are positive, but when using Fn to find friction it is actually |Fn| because a direction vertically does not correspond to a direction horizontally. Finally the direction of friction opposes the motion, so if you try to move something right, the force of friction will be to the left.

[hamid14]

oh ok, cause im calculating mu for horizontal motion only, not vertical motion. horizontal motion is positive either way? for left, it wouldnt be negative? our teacher doesnt teach us how to calculate stuff, we have to learn on our own btw. >.>

[SNIPERDUDE]

It would be a positive number, but closer to zero.

[hamid14]

thanks a lot guys! got the answer >.>

[TerranceN]

Mu doesn't change no matter the direction of the normal force, it is a constant based on what two materials are touching each other (ex. mu will be the same if you slide something wood across a wooden floor and if hold something wood against a wooden wall).

Edit: Misread the post, thought you meant the direction of friction, I'll just leave this here anyway.

Also, I will be more specific from now on, |Ff| = μ * |Fn|, you need to add the direction on your own. Since you can set any direction to be positive, if you set right as positive and the applied force is to the right (therefore friction would be to the left), in vector calculations (the ones where direction matters) friction will be negative.