# Measuring the Coefficient of Friction – Updated

Posted December 22, 2010

on:This is a follow up on the first pass of a lab I created about a year ago. You can see my first post here. This year we purchased a whole pile of new technology including the LabQuest from Vernier. If you have these in your lab, you know just how cool they are. The kids love technology, this lab grabbed their interest.

When I came up with a lab last year, we used spring scales to attempt to measure the coefficients of friction on all different surfaces. Let’s face it, doing this with a spring scale was mostly guesswork. I enhanced the lab with the digital force meter, made a few quick modifications to the instructions and gave it to the kids. The results blew me away.

The graph you see below was printed directly from the LabQuest. I tried exporting it as a file, but the export file wasn’t an image file, so I scanned it for all of you to see. Click on it to see it full size. The red line shows the force required to pull a painted steel plate across linoleum. You can clearly see the spike where the pulling force overcomes static frictional force. The next horizontal data set shows that the object was pulled at a constant speed. We can read both the static and kinetic forces directly on the LabQuest.

I included a shrunken image of this graph on today’s test. I gave the students the weight of an object and they had to calculate the two coefficients of friction from the graph.

Here is the handout: Measuring the Coefficient of Friction – LabQuest, feel free to steal it and use it. If you have ideas to improve it, leave me a comment.

Here is an interesting finding – shoe rubber seems to have a static coefficient that is the same as the kinetic coefficient, pretty much regardless of the surface. On linoleum, it’s around 0.60.

### 4 Responses to "Measuring the Coefficient of Friction – Updated"

Hi Scott,

One pedagogical change I have implemented in my physics teaching is having students do labs in order to discover the relationship on their own, rather than after the fact.

We just did our friction lab and we used a discovery approach. I should be blogging about that soon.

Anyway, there are a bunch of great physics bloggers out there sharing ideas. I don’t know if you know of these folks since they aren’t on your blog roll, but they are FANTASTIC!

John Burk

http://quantumprogress.wordpress.com/

Shawn Cornally

http://101studiostreet.com/wordpress/

And you can find a bunch more on the blog roll on my blog:

http://fnoschese.wordpress.com/

Sharing ideas can only help us all get better! Keep in touch! Get on Twitter if you can!

-Frank Noschese

We currently do a friction lab with spring scales. It’s true that the measurements aren’t super precise, but I have students measure frictional force for different normal forces and produce a graph. Each group gets a different surface so we see the linear relationship and “discover” that the slope can describe the frictional coefficient (kinetic). I like doing it this way because my students need more practice graphing dependent vs. independent variables and interpreting mathematical relationships.

1 | Marcie

December 23, 2010 at 4:07 pm

Scott,

Glad to see you are surviving your year of new preps.

I do a similar lab but instead of changing surfaces, I change sides of a block so the kids can see that surface area does not affect mu.

This year I added another part which went brilliantly. I cut up pieces of a tire and had then figure out mu on pavement. Then they discussed all of thier finidings as an expert in a reckless driving trial (using forces to find accel. and skid marks etc to fill out the rest of the kinematic equations to evenutally find the initial speed of the driver). They had to convey all of thier finding to the judge including why they were able to use a section of tire to represent the whole thing, how they found friction, what friction depends on, etc. It was a wonderful exercise for them to really have to explain everything.

Scott

December 24, 2010 at 12:10 pm

Honestly Marcie, I’m just barely surviving, that’s why you hardly see any posts this year. The workload kind of comes at me in tidal waves. Robotics/Astronomy has shifted to Astronomy and I need to put the course together some time soon. Calculus has moved past derivatives and I need to spend some time in the textbook relearning the important points. Physics is just constant quizzes, tests, and labs. It is all going well, but next year will be a lot easier.

I’d like your handout from the crash analysis. That sounds like something a couple of my students might be able to handle, and it sounds like a cool project.