# Homework Help: Coefficient of friction ?

1. Dec 17, 2012

### huymai

Coefficient of friction!!?

1. The problem statement, all variables and given/known data
How would you figure out the coefficient of sliding friction between two sheets of paper ?.

2. Relevant equations

3. The attempt at a solution
I intended to make a ramp out of one paper and and put the other paper on top of the ramp. Adjust the angle until the paper slide down. Take the tangent of the angle when the paper slide down to find the coefficient of friction.
Is this the right way to approach this problem?

Thank you,

2. Dec 17, 2012

### CWatters

Re: Coefficient of friction!!?

Perhaps repeat many times and calculate an average? Check the spread of results to see if they are repeatable or affected by something?

How using a stack of multiple sheets to ensure a uniform contact?

Edit: Do you want to measure the coefficient of static or dynamic friction? That method is ok for static friction.

3. Dec 17, 2012

### swsw

Re: Coefficient of friction!!?

By your method in 3, I am assuming that you want to measure the static friction coefficient between the two paper.
I have no idea what type of ramp you intend to make (wood with paper surface or entirely out of paper), whatever it is, make sure the paper (the sliding one, not the ramp) is heavy enough at the top. (I suggest pasting the paper onto a heavy object with a flat surface such that there is enough contact force and the paper will not fly off.
F=μkN, N is the normal force on the sliding object. With the additional weight, N can still be accounted for.
Take a few readings to ensure accuracy. Try to do the experiment in the absence of wind or any external force.

4. Dec 17, 2012

### PhanthomJay

Re: Coefficient of friction!!?

As CWatters mentioned, determining the tangent of the angle where the paper covered object just starts to slide down the paper covered ramp will give you the static friction coefficient. To get the kinetic friction coefficient, you need to determine the angle where the paper covered block slides down the ramp at constant speed (no acceleration). How might you do that? Will that angle be greater or less than or equal to the static friction angle of the plane?

5. Dec 18, 2012

### swsw

Re: Coefficient of friction!!?

Perhaps it is possible to use a heavy object with a paper surface (as suggested before) and also a wooden plank with a paper surface.
Place the object with paper surface on plank, slowly lift plank to the angle whereby the object first starts to slide.
Use trigonometry to find the angle, you can measure the length of the plank and the height the tip of the plank is from surface of table or floor.

6. Dec 18, 2012

### PhanthomJay

Re: Coefficient of friction!!?

Which gets you the static friction coefficient, but not the sliding friction coefficient, which can be determined in a bit different way somewhat similar to this, and i would like for the OP to express some thoughts on how to go about this.

7. Dec 18, 2012

### swsw

Re: Coefficient of friction!!?

I see, I thought huymai meant static friction coefficient as written in part 3.
If it is kinetic friction coefficient, then there is no need for the ramp. just connect the heavy object (with paper surface) to a spring balance and pull. Take the reading of the minimun force required to keep the object in motion and not the initial thug to start the motion (across a paper surface).

8. Dec 18, 2012

### haruspex

Re: Coefficient of friction!!?

Another way is to find the slope needed to overcome static friction, then try slightly smaller slopes. In each try, give the block a nudge and see if it keeps going.
Yet another: block on rotating turntable, block restrained by spring balance.