 #1
 33
 4
Homework Statement
So in physics we have this lab. Its simple, I'm just bad at physics...so bare with me.
The objective of the lab is find the coefficient of friction (on a flat surface not on inclined planes and such not sure if this makes a difference (I should know if it does but I don't and so I will say as clarification, that we only did this experiment on level surfaces)). I need help with the procedure for this. So what our group did was:
As instructed: get a wooden block with a known mass (40g > 0.04kg) and get several cylinders with varying (known) masses:
Record their individual mass and weight and then their combined mass and weight.
The cylinder masses we used (for each surface the experiment is run several times with different cylinder masses) were
Mass: 0.5 kg the weight> 4.9 N
Mass: 1 kg the weight> 9.8 N
Mass: 0.2 kg the weight> 1.96 N
Mass: 0.05 kg the weight> 0.49 N
Mass: 0.1 kg the weight> 0.98 N
and since our blocks mass was 0.04 kg ; the weight being 0.39 N, our total weight for each trial (per surface) is as follows:
5.29 N
10.19 N
2.35 N
0.88 N
1.37 N
~~~~~~~~~~~~~~~~~
Our first surface was our desk. So, we hooked the block with the cylinder mass on top (starting with 5.29 N as our combined total weight) to a spring scale to measure F_{app} . On our lab worksheet it is listed in the table where our measurements were recorded, that F_{app} = F_{f}
So using the spring scale and dragging the block and cylinder across a flat surface, we recorded our F_{app}, for all of the total weights.
For the Table:
Total weight (F_{n}): 5.29 N F_{app}> 2 N
Total weight (F_{n}): 10.19 N F_{app}> 2.8 N
Total weight (F_{n}): 2.35 N F_{app}> 0.8 N
Total weight (F_{n}): 0.88 N F_{app}> 0.2 N
Total weight (F_{n}): 1.37 N F_{app}> 0.4 N
For a piece of Cardboard:
Total weight (F_{n}): 5.29 N F_{app}> 1 N
Total weight (F_{n}): 10.19 N F_{app}> 2 N
Total weight (F_{n}): 2.35 N F_{app}> 0.4 N
Total weight (F_{n}): 0.88 N F_{app}> 0.1 N
Total weight (F_{n}): 1.37 N F_{app}> 0.2 N
For a piece of Styrofoam:
Total weight (F_{n}): 5.29 N F_{app}> 1.4 N
Total weight (F_{n}): 10.19 N F_{app}> 2.6 N
Total weight (F_{n}): 2.35 N F_{app}> 0.6 N
Total weight (F_{n}): 0.88 N F_{app}> 0.15 N
Total weight (F_{n}): 1.37 N F_{app}> 0.25 N
We used the same weight combinations for each surface. And experimented with 3 surfaces.
Homework Equations
[/B]
F_{n} = mg
F_{f} = μk*F_{n}
The Attempt at a Solution
With F_{app} being stated on the worksheet as equal to F_{f} we used the equation:
F_{f} = μk*F_{n}
We divided both sides by F_{n} so that :
F_{f}
 = μk
F_{n}
So for the Table we got:
2 N
 = 0.38
5.29 N
2.8 N
 = 0.29
10.19 N
0.8 N
 = 0.34
2.35 N
0.2 N
 = 0.23
0.88 N
0.4 N
 = 0.29
1.37 N
For the Cardboard:
1 N
 = 0.19
5.29 N
2 N
 = 0.20
10.19 N
0.4 N
 = 0.17
2.35 N
0.1 N
 = 0.11
0.88 N
0.2 N
 = 0.15
1.37 N
For the Styrofoam:
1.4 N
 = 0.26
5.29 N
2.6 N
 = 0.26
10.19 N
0.6 N
 = 0.26
2.35 N
0.15 N
 = 0.17
0.88 N
0.25 N
 = 0.18
1.37 N
For the Table:
See how different all of those μk's are??? Ahh :''
It was at this point we asked our physics teacher, not exactly for an answer, but for any advice. What he gave us was as such:
"When I apply to a job I have to submit paper work
When I apply (insert example I have to (insert relevant, corresponding factor)
When you apply force you have *pause* do something. That's as big of a hint as I can possibly give you"
I still don't know what he meant. I've thought about all the things I could possibly be doing that, as of current, were not done. I've come up with several things maybe but I think they are not exactly relevant, and I'm just reaching past what I need to into something unnecessary. I don't know which is why I consult the friendly users of PhysicsForums. I will finish listing my data in spoilers before I pose my final question.
For the Cardboard:
Closer than the first set of data, but still to distant to be reliable. There is still a very obvious flaw...one I cannot myself identify.
...and now my n key is stuck :' (rip)
For the Styrofoam:
As you can see with this last set of data, on the Styrofoam, three resulting μk's all had the exact same value. I would like to believe this to be a sign that our procedure was correct ad that inaccurate results previously can be attributed solely to massive human error, which will be corrected later in a redo experiment if that is the case.
My confusion is simply trying to understand whether or not, it was only human error responsible for our drastically varying results, or our procedure was flawed. Like our teacher said, did we have to do more with F_{app}? Was it not directly equivalent to F_{f}, despite the worksheet saying as such? What should we have done instead if our procedure contained the inaccuracies? For once I can't be led with hints to find the answer myself. I've evaluated every aspect and I myself cannot come up with a solution so simple, clarification instead of hints would be greatly appreciated.
Attachments

11.1 KB Views: 264

1%80%D0%BC%D1%8B%D1%80%D0%B0%D0%B2%D0%BD%D0%BE%D0%B3%D0%BE%D0%BE%D0%B1%D1%8A%D0%B5%D0%BC%D0%B0.jpg1.9 KB Views: 322
Last edited: