# Homework Help: Friction between two surfaces

1. May 13, 2007

### jamesyboy1990

1. The problem statement, all variables and given/known data

hi, i am almost done my lab on the coefficient of friction on an inclined plane. I was attempting to prove that the coefficent of static friction would be the same no matter the weight of the mass that was static on the incline. However, i plotted a coefficient versus mass graph, and the line wasn't horizontal (the coefficient changed). My quick question was whether i still need to find the equation for finding the coefficient based on my results, or if i could just conclude that in my case, the coefficient didn't stay constant, and then move to my conclusion

2. Relevant equations

3. The attempt at a solution

2. May 13, 2007

### Hootenanny

Staff Emeritus
How did you calculate the coefficient of friction for your results?

3. May 13, 2007

### jamesyboy1990

i had an incline, measured the weight of the mass, and changed the angle such that if i increased the angle by a little bit, the mass would slide. I did this for several different masses and recorded the angle. I then used a freebody diagram and the equation for static friciton (u = Fmax/R)

4. May 13, 2007

### Hootenanny

Staff Emeritus
And how did you calculate the normal reaction force?

5. May 13, 2007

### jamesyboy1990

R = mg cosx
F = mg sinx

so in essence, the coefficient was tanx

6. May 13, 2007

### jamesyboy1990

technically, all i need to know is whether i would have to process the graph to find a relation between weight and the coefficient (although i know that technicallly the coefficient is independant of mass) if all i wanted to prove in the lab was that the coefficient was independant of mass (and in my case, my experimental data didn't prove it)

7. May 13, 2007

### Hootenanny

Staff Emeritus
Your not actually plotting $\mu$ vs. $m$ what your actually plotting is $\tan\theta$ vs. m since;

$$\mu = \frac{F}{R} = \frac{mg\sin\theta}{mg\cos\theta} = \tan\theta$$

Last edited: May 13, 2007
8. May 13, 2007

### jamesyboy1990

but since tanx = u, so why should it matter whether you plot u vs. m or tanx vs m?

9. May 13, 2007

### Hootenanny

Staff Emeritus
Indeed, $\tan\theta = \mu$, but are you keeping the angle constant?

10. May 13, 2007

### jamesyboy1990

no, because the greater mass wouldn't stay static at the certain angle, so it would decrease.

if i am trying to prove that the coefficient should stay the same, and it doesn't in my experiment, do i have to do more, or can i just stop once the graph shows that the coefficient changed?

11. May 13, 2007

### david1701

i thought that: Frict max=mu R
so for your experiments to prove mu is irrespective of mass the particle has to be on the point of slipping
that may be too simple i am only doing as physics and you guys might be talking about something higher tho from mechanics mu is >= to R/Frict max
(sorry about symbols ect)

12. May 13, 2007

### jamesyboy1990

honestly, the only question i wanted answered was in respect to the set up to the lab. Once you process enough data to prove/disprove the purpose of the lab, do you have to do any more processing, or can you just go straight to conclusion/evaluation?