Friction between two surfaces

1. jamesyboy1990

16
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. Hootenanny

9,681
Staff Emeritus
How did you calculate the coefficient of friction for your results?

3. jamesyboy1990

16
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. Hootenanny

9,681
Staff Emeritus
And how did you calculate the normal reaction force?

5. jamesyboy1990

16
R = mg cosx
F = mg sinx

so in essence, the coefficient was tanx

6. jamesyboy1990

16
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. Hootenanny

9,681
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. jamesyboy1990

16
but since tanx = u, so why should it matter whether you plot u vs. m or tanx vs m?

9. Hootenanny

9,681
Staff Emeritus
Indeed, $\tan\theta = \mu$, but are you keeping the angle constant?

10. jamesyboy1990

16
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. david1701

4
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. jamesyboy1990

16
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?

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