Friction Experiment: Understanding Necessity & Sources of Error

[chrisfnet]

Homework Statement

We recently performed an experiment to determine the static and kinetic coefficients of friction by moving a box along a board. In some instances we used a pulley and weights to find the static and kinetic coefficient of friction and in the last part we inclined the board and gave a slight push until the box slid down at a constant speed.

At the end of the experiment, he's asked us three questions. I've got answers to two of the three (could someone look at those and give critique, or suggestions for addition?) The first question is the one I'm having a little trouble with.

The Attempt at a Solution

Why is it necessary for the string to be level in parts one and two?

I'm guessing if the pull (string) is horizontal, the normal force would then equal the weight of the box.

But I don't know what happens exactly if the string isn't level...

Why should the coefficient of kinetic friction be less than the static one?

It takes more force to overcome friction when the object is at rest than it does to keep the object moving.

Also, when an object is at rest the two surfaces have a chance to fully interlock on the microscopic level. When the object is moving, only macroscopic irregularities can interconnect.

What are other sources of error?

One source of error is that we may have been accelerating the box, rather than applying a force only sufficient to overcome friction. Despite observing what seemed to be constant motion, the box may have actually been accelerating.

Another source of error is the lack of small incremental weights. We had to use larger weights and in most instances we think we were only accurate +/-10g. This introduces a pretty significant margin of error.

We also ignore the contribution of friction by the pulley and the string.

 

[turin]

For the first question:
Consider your fbd's. What equations do you get from them? What are the variables that you assign from experimental input, what are the variables that you assign from experiemental output, and what are the unknowns that you are trying to find? How does the angle of the string from horizontal affect these equations, and confound your solution for the unknown? You should try to understand this experiment in terms of the techniques that you have learned to solve the textbook friction problems.

For the second question:
What is the physical meaning of a coefficient of static friction that is less than the coefficient of kinetic friction? How would you experimentally determine its value? Remember, the friction coefficients are phenomenological.

 

[chrisfnet]

So if the string isn't horizontal/parallel to the board, the normal force is changed and the whole acceleration of the system is changed as well. If the string were too "high", wouldn't this reduce the friction to some degree?

 

[chrisfnet]

I'm still really confused about this.

So, I'd have the normal force and friction force, tension force and mg (mass/gravity = weight) force all in the FBD.

If the string were not level, the normal force would not necessarily be perpendicular to the plane. In the case that the string were too "high" at the pulley, friction would be reduced and if the string were too "low" at the pulley, friction would be increased?

 

[turin]

Yes, I believe that you are correct. (Note: there are three components of tension for the three spatial dimensions. The two that are parallel to the table will be independent of the normal force. However, the component perpendicular to the table will have an effect on the normal force, which in turn would have an effect on the friction, as you suggest).