# Homework Help: Static friction co-efficient

1. Nov 11, 2008

### yoleven

1. The problem statement, all variables and given/known data
from the free body diagram of a block resting on a flat surface, I am trying to derive the coefficient of static friction.

2. Relevant equations

3. The attempt at a solution
I have the definition of the coefficient as:
us=Fn/Ffr

But in my free body diagram i am pulling on the block, which resists with the Ffr
I have
$$\Sigma$$Fx=0
F-Ffrus=0
us=F/Ffr

My confusion is because if the definition of the coefficient is above, why don't I derive it when I observe my free body diagram of the block. What am I missing?
The forces in the x direction are my pulling force and the friction force that resists it * us
How do i get the weight of the block into my derived equation?

2. Nov 11, 2008

### Staff: Mentor

The weight of the block is Fn.

(If that's not what you're looking for, please state the complete problem exactly as it was given.)

3. Nov 11, 2008

### yoleven

It is not a problem as such. I am trying to determine the coefficient of static friction us experimentally. Then, I am trying to determine if it is a function of surface area or of mass.
My experiment consists of a wooden block on a wooden flat surface. I am going to attach a spring scale and determine at what force the block overcomes the friction and moves.

In trying to determine us from my free body diagram I am having a little difficulty as I tried to explain.
From my free body diagram, if at equilibrium the forces in the x plane are zero, I get
Fpull-Ffriction*us=0
but I don't see where I am getting the mass of the block to become part of my derived equation.

4. Nov 11, 2008

### marlon

you also need to apply newton's second low in the Y-direction (vertical direction)

ma_y = F_normal - mg = 0

do you understand this equation ? Why is it equal to 0 ?

ps a_y is the component of the acceleration in the vertical direction
and F_normal is the normal force.

In this case, this is not gonna help you much because the table is horizontal.

marlon

5. Nov 11, 2008

### Staff: Mentor

What you should get is Fpull - Ffriction = Fpull - μFnormal = 0.

6. Nov 11, 2008

### yoleven

Thank you.

7. Nov 11, 2008

### marlon

Nope,

Fpull=Fnormal*us