I have been doing a lab for my class and before we started the lab we wiped our surfaces (rubber and slate [is a lab bench so it has been fireproofed]) with a wet paper towel and dried it off quickly. We did calculation and found the coefficient of kinetic friction to be far too high (over 1.5) so I think the water may have played a role in the wrong calculations. What is the general effect of wet surfaces on the coefficient of kinetic friction of objects because many sources say that water acts as a lubricant, but I noticed that it seems to decrease the acceleration of objects. In this experiment I did not just leave the surfaces wet, but rather I dried them off so it was more akin to cleaning but the results seemed to show a massive increase in the coefficient of friction.
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$\begingroup$ Let the water COMPLETELY dry before running your experiment. Just because it LOOKS dry doesn't mean it IS. Setup a fan & heat lamp or some such to make sure. Then take your measurements. The idea that 'water is always a lubricant' is definitely not true - There are many glues that start working once water is added (Not saying that the benchtop is glue-like... just making a point that assumptions should always be questioned). $\endgroup$– Kyle BCommented Feb 15, 2022 at 6:24
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Generally water greatly decreases coefficient of friction between 2 surfaces. That is why you should't drive car fast when raining. How did you do the measurments? Perhaps you made some systematic error?
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$\begingroup$ I used an Atwood's Machine and used displacement, time and initial velocity to calculate the acceleration of the system. Once I had the acceleration I used the equation mu kinetic = (mB*g - a(mB+mT))/(mT * g) with mB as the mass on the bottom of the Atwood's machine and mT as the mass on the top of the Atwood's machine. This means that I assume that any forces acting against the motion of the block are negligible except for kinetic frictional force. $\endgroup$– LitGamerCommented Feb 15, 2022 at 0:53
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$\begingroup$ If you used the Atwood's machine for measuring coefficient of friction then firstly for your calculations to be valid there should be no rotatoon of the block. Secondly what did you consider as normal force in calculations of coeficient of friction? $\endgroup$ Commented Feb 15, 2022 at 1:01
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$\begingroup$ Normal force in the calculations is the weight of the object at the top of the machine and the block did not rotate during the experiment. I noticed that cleaning surfaces drastically increase the coefficient of friction, but it still seems unreasonable to have a coefficient of kinetic friction close to 2 for rubber and slate. $\endgroup$– LitGamerCommented Feb 15, 2022 at 1:05
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$\begingroup$ This is obviosuly an error. The normal force is due to tension of rope. Cleaning the surfaces reduces coefficient of friction. Otherwose all machines would be dirty. That's nonsense! $\endgroup$ Commented Feb 15, 2022 at 1:08