Coefficient of friction different for different angles.

[Hlud]

I am having my students do a lab to calculate the coefficient of static friction along a ramp. They are calculating the coefficient of static friction between felt and the table to be something like 0.40, yet when i did this lab, but along a horizontal surface, and not with a ramp, i get a coefficient around 0.25. Am i making a wrong assumption here, or am i making a lab error.

 

[nasu]

How did you measure it, for each of the two experiments?

 

[Hlud]

How did you measure it, for each of the two experiments?

For the first experiment, we dragged a block along the table, ever so slightly increasing the applied force (which is attached to a forcemeter) until it started moving. We plotted the graph, which clearly shows the friction graph, and calculated mu from there.

For this year's experiment, we increased the angle of the table, ever so slightly, recording the angle from our phones, until it just starts to slide. We then calculate mu using mu = tantheta.

 

[sophiecentaur]

The formula for friction coefficient involves the 'normal Force'. Was that the same for all experiments?

 

[nasu]

So the two surfaces were different, weren't they? The ramp is something else than the table, I understand.
And you did these experiments with one year interval in between?

 

[Hlud]

So the two surfaces were different, weren't they? The ramp is something else than the table, I understand.
And you did these experiments with one year interval in between?

No, they were the same surface. And yes, the data was from last year.