# Day 41: Teasing Out Friction’s Dependence on Normal Force

College-Prep Physics: Last class, students determined the factors that affected friction. They had said one of the factors was mass and/or weight. Today, we dug a bit deeper and analyzed a few different scenarios (above) to tease out the real factor. They did fine drawing the force diagrams (my annotations in red):

We skipped the interaction diagram this time, since I figured the scenarios were fairly easy. However, a few groups ended up drawing a single combined downward arrow for C and a single upward arrow for D, rather than 2 arrows to represent each object interaction.

Then I asked the class, “For your ranking, which value from the force diagram aided in your ranking?”

It’s not really about weight or mass. It’s not really about the downward force. It’s about the upward force from the surface!

Then we tried using our interlocking bristles model to explain our predictions. The more the surfaces are compressed together, the more the surface bristles interlock, and therefore the more friction there will be.

##BFPM

NGSS Science and Engineering Practices
#2. Developing and Using Models

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#### About Frank Noschese

HS Physics Teacher constantly questioning my teaching.