Factor of friction strange behavior

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Hello everyone, I have a really interesting question here. I am measuring the factor of friction between two different metals. My results are a beat cloudy. When I use Steel - Brass surfaces the factor of friction, μ is being reduced while I am increasing the applied force. In other side, when I use Steel - Aluminum surfaces, when I am increasing the applied force on the two materials the factor of friction, μ goes up. Anyone can explain what is happening? There is no measurement mistake, that' s guaranteed.
 

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Hello everyone, I have a really interesting question here. I am measuring the factor of friction between two different metals. My results are a beat cloudy. When I use Steel - Brass surfaces the factor of friction, μ is being reduced while I am increasing the applied force. In other side, when I use Steel - Aluminum surfaces, when I am increasing the applied force on the two materials the factor of friction, μ goes up. Anyone can explain what is happening? There is no measurement mistake, that' s guaranteed.
You'll have to tell us a bit more: What are the magnitudes and ranges of normal forces that you're working with, how are the friction surfaces prepared, what pressure is involved, how is the normal force applied and how is the coefficient of friction measured? There are a lot of ways that your experimental setup could deviate from the ideal conditions in which the coefficient of friction is close to independent of the normal force.

(Also "no measurement mistake, that's guaranteed" is something that we've all regretted saying at least once. Just sayin' :smile:)
 
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(Also "no measurement mistake, that's guaranteed" is something that we've all regretted saying at least once. Just sayin' :smile:)
One of our lab tutor told as so as, we aren' t supposed to focus on such a thing and focus on what' s going on :smile: I can understand you objections on that.

Now with the experiment part:
Our experimental setup includes a steel rotation disk, and to measure the friction factor we use some aluminum and brass rods of the same radius. We use pressures of 1, 2 and 3 bars which creates forces of about 106 N, 212 N and 318 N in our experimental setup.
The surfaces are being threw a high standard rectifier machine, which makes them really smooth. The forces are vertically applied on the rod. The coefficient of friction is measured by a electronical device which gives as the force of the friction. We use the type μ=T/(2*F) as we used two rods at the time, at the opposite side of the disk.

F: Applied Force
T: Friction Force
μ: Friction Factor
 
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