Differences in kinetic friction coefficents

[jmcmillian]

Is a difference in .07 when it comes to coefficients of kinetic friction for the same object on the same surface significant? I thought it was, but I have yet to determine a reason why. Below is an explanation of my experiment, as brief as possible.

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I ran an experiment in which I had a wooden block and a weight.

My objective was to determine the coefficient of kinetic friction for the block and the weight.

I did this using two methods: One in which the block+weight was tied to a sensor box, which was connected through an interface to software. As I diligently pulled the sensor box, the block+weight would move (overcoming static friction) and then both units would slide across the table as I pulled, only facing kinetic friction. I did this four times, and received similar results on each.

The other method I used was with a motion detector apparatus. I took the block+weight and simply gave it a shove towards the detector. I did this five times, and saw similar results.

Now my problem is that my coefficient of kinetic friction for the push is on average .07 more than my value for the pull.

My first attempt to rationalize this is that the force I applied in the shove (push) was much greater than the force for the pull, but I then reasoned that it shouldn't really affect the kinetic friction coefficient, only the static value.

I ruled out a anomaly in the average of each because the results were precise...
< .02 or less difference in each of the 5 trials on the push experiment
< .03 or less difference in each of the 4 trials on the pull experiment.

Any ideas on how to explain this?

 

[hunter151]

Are you pulling the block absolutely horizontally? You may have some force going into lifting or pushing the block down which would cause your normal force to be less/greater than what you calculated, resulting in a different coefficient of friction.

 

[jmcmillian]

Are you pulling the block absolutely horizontally? You may have some force going into lifting or pushing the block down which would cause your normal force to be less/greater than what you calculated, resulting in a different coefficient of friction.

hunter, Thanks for the advice. There might have been a slight angle to me holding the rope, resulting in the difference.

 

[hunter151]

Try calculating the angle you would have to make to account for the difference in the the coefficients and see if this is a plausable angle.