# Lab - Friction and Forces on an Incline

1. Mar 26, 2009

### soccergirl14

Okay, so I am writing a lab but I am quite stuck on one question.

Using the average force of sliding friction from the data, calculate the coefficient of sliding friction.
Average force of sliding friction: 0.32N

The equation I used to calculate other static/sliding coefficients was:
(FgSinX)/(FgCosX)
And X is the value of an angle.

I am confused because I don't know if I can use this formula to calculate the coefficient because the average force of sliding friction is not an angle.

If I use the average force of sliding friction as an angle I get:
(9.8xSin0.32)/(9.8xcos0.32) = 0.006
This does not seem right, anyone have any ideas?

2. Mar 27, 2009

### LowlyPion

Don't you want to look to the definition of μk?

μk = Ff/Fn

If you have determined Ff, then aren't you just interested in dividing by the Fn ?

3. Mar 29, 2009

### soccergirl14

Okay, thank you, I think I understand.
What I tried was:
(0.32N)/(9.8cosX)
Would that make sense?

4. Mar 29, 2009

### LowlyPion

Not quite.

The normal force is given by cosθ * weight, but there is also the component of gravity down the incline of sinθ * weight. If the force you are measuring pushing up the ramp is .32N then isn't that the result of the relationship that

.32 N = μ*m*g*cosθ + m*g*sinθ

If .32N is measured pushing down the ramp then

.32N = μ*m*g*cosθ - m*g*sinθ

5. Mar 29, 2009

### soccergirl14

Okay I get it!
thank you so much for the help!