On the Surface

Interactions between two surfaces, or coefficients of friction, has been a difficult topic for some of my physics students.  Thanks to online resources from The Physics Aviary,  Vernier Scientific, and The Physics Classroom, I found a sequence of activities that really made this concept crystal clear to my students and that included a very timely real-world application.

We usually start with a lab to determine the coefficient of static and kinetic friction between two surfaces by using a block of wood and spring scale.  This year, I used the Physics Aviary’s Friction Lab simulation that “uses” a Vernier Dual Range Force Sensor and a Labquest 2.   Students “pull” a block (with the force probe) across a surface.  As the tension increases, the block begins to move.  The block then moves at a constant speed.  A graph of tension vs time is produced.  Using this data and the normal force of the pulled block, students can determine the coefficients of static and kinetic friction.  Students can also change the size of the block, mass of the block, and the types of surfaces in contact with each other with this simulation. 

After students understand how to determine coefficients of friction, we do a “Mu of Your Shoe Lab” using the same type of equipment from the simulation.  Students determine the coefficient of friction for various types of shoes.  The Physicsclassroom.com has a nice version of this lab.  Vernier Scientific does a helpful video about this lab.  The video is designed for the middle school version of this lab, but there are some great tips for this lab.  When my students did the “Mu of Your Shoe Lab” after the simulation, there was less confusion about the procedure and the lab produced better results.

I added a final component to the lesson this year.  Any discussion of coefficients of friction always include those of various types of road surfaces. We had an early snow this winter and a record-breaking amount of rain this year, so talking about different driving conditions was very relevant!  The Physics Aviary has a simulation that brings the stopping distance for a skidding car together with the mass and initial speeds of the car, and different types of road surfaces.  Students study how each of these variables affect the stopping distance of a skidding car.  A link to the Hyper Physics Friction and Automobile article was added to prompt student reflections about the lesson sequence.