## Question on friction

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xepher
Posts: 83
Joined: Tue Mar 09, 2010 1:42 am UTC

### Question on friction

So I'm in your average early physics lab, and we're going to find an approximation to the coefficient of friction (in this case, static friction) between two surfaces.
The method we're using is that we are finding the angle of repose for this object, and using that to find the coefficient. I just have one problem with it...
Mu is the tangent of the angle of repose, right?
So we change the value of the mass for one of the objects, although the surface remains the same. So, mu should be the same, right? Well, apparently not. When we did this, the angle seemed to go a tad higher with the more mass. I don't get it, since the coefficient of friction should be constant.

Is this supposed to happen or did my team and I just change the angle so that it was like that?

khanofmongols
Posts: 21
Joined: Wed Apr 21, 2010 2:43 am UTC

### Re: Question on friction

The coefficient of friction isn't perfectly constant for all normal forces and velocities but is mostly constant. At higher normal forces the coefficient goes up because the uneven surfaces mesh more. Also at higher velocities the coefficient of friction decreases because the surfaces "skip" off of each other.

Dopefish
Posts: 855
Joined: Sun Sep 20, 2009 5:46 am UTC
Location: The Well of Wishes

### Re: Question on friction

Also, don't worry 'too' much when you get rediculously 'wrong' results in a (first, maybe second year) lab course, as long as everyone else is getting similar. I've had labs where errors of +/-100% was expected of us . In my experiance, they're mainly looking for you to come up with good explanations for the sources of error and make you think about things, as well as get used to the ideas surrounding the procedure(s), rather than actually getting decent results.

Of course, that doesn't mean you shouldn't try your best and take repeat measurements of everything in an effort to minimise error, and it's good to be concerned about results that seem to contradict theory, but don't let the results get to you too much as long as you can explain the errors.