Force of Friction

On a Monday last spring I realized that the week five laboratory in physical science had to tackle friction. Throughout week four and into week five I find myself saying to the class, "But for friction, the results would agree with the theory..." I blamed friction and then never had the class further explore friction.

Maxon gathers data

As reported in a blog, the laboratory was run as an ad hoc open exploration. Assessments after the laboratory, however, demonstrated that the students were confused as to the possible meanings of their results. The need to control two of three factors eluded the students. The idea of varying one variable while controlling other variables was not only a new concept, but a non-obvious concept for the students.

Initial data on sandpaper grit versus pulling force

The one plus was an attempt in spring 2016 to provide some closure and information exchange by having the students present results to each other the next day. The decision to run an impromptu workshop on Friday was the serendipitous arrival of a workshop invitation in my email on that Thursday afternoon back in spring. That basic structure will be retained this term.

Tania, Stewart, Ray, working with the triple-stacked surface area sled stack

The structure of the summer term did not permit time for presentations - the laboratory remained confusing for the students and the laboratory period was consumed with sorting out the variables. The laboratory report was also a complex beast of at least three tables and three graphs, with some groups producing four and five tables and graphs. There was, for each group, an issue of information overload. Too many variables, too much data, hard to see the forest for the trees.

Francine, Ashley

Given that the laboratory had not proven illuminating of the nature of friction for the students in the spring and the summer terms, I opted to restructure the laboratory.

Osbert, Twain

Each pair tackled only a single variable. The three variables, weight, surface area, and grit, were pre-identified at the start of class. This meant that the laboratory was no longer an open ended data exploration exercise, but a more structured laboratory.

Gayshalane, Francine

Each pair explored either weight, surface area, or grit. The pairs measuring the effect of weight used the same sled for all measurements, and a single sheet of sandpaper that varied in grit from pair to pair. The sleds weighed on the order of 200 grams, some more, some less. The weights included a couple of 50 g, 100 g, 200 g, and 500g. Spring scales measured the force of kinetic friction. Although I mentioned the two types of friction, the laboratory focused on kinetic friction.

The grit pairs had 60, 120, 360, 600, and 1500 grit sandpaper to work with, a single sled, and a single sheet of sandpaper.

In each section only a single group tackled surface area using three pairs of back-to-back glued glass sleds each with a pull string in the middle. This provided up to five measurements (at least two of the sleds have effectively identical surface areas). The sleds were always stacked and pulled as a stack, which provided a constant weight. The grit was not changed during the experiment. Although a prismatic sled had been designed for use last summer, during class that summer I realized that one can simply stack the sleds on each other and get more surface areas with the same weight.

Data above shows a positive relationship between weight and force. Each pair was told to be prepared for an oral presentation the next day, Friday.

Grit shown to potentially not related to force, at least not in the initial run of measurements

Glenn, Vandecia

The new structure simplified each pair's task and leads to a single table and graph for each pair. Each pair did appear, for the most part, to generate data that reflected theory (the students were not exposed to any theory prior to the laboratory). The presentations should, if they go well, help tie together the bigger picture of which variables have the most impact on the force of kinetic friction.

Nagsia, Michelle

Yuta, Beverly-Ann

Nagsia, Michelle

One group which had originally generated data suggesting no relationship between grit and force went back, remeasured, and found a relationship between grit and force. I had the sense that they felt compelled to find a relationship - a finding of no relationship seemed to be insufficient for the group.

Anster, Madlina

The surface area data must be multiplied by a factor of ten, the width of the glass louvers.

On Friday the student presentations went very well. 

Nagsia and Michelle presenting on Friday

The groups measuring the effect of weight had all printed out graphs of their data, and the data was exceptionally linear. Five groups measured the effect of weight, with slope values of 0.4, 0.354. 0.31, 0.4, 0.4. This value is unitless, a coefficient of friction for glass. 

The two groups who worked on surface area found no relation, with one group finding a frictional force of 300 grams for all surface areas. Their sled stack was a 983 gram stack. That ratio implies a coefficient of 0.31, which concurs with the results of the weight groups.

The groups working grit split. Two of four groups found no relation, the other two asserted a linear relationship with slopes of 0.0249 and 1.4828 grams per grit unit. Grit was simply the grade of the sandpaper. 

The Friday presentations provided a strong close to the week. 

An attempt to use the new Chromebook in A101 failed due to WiFi signal strength being too low.


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