Day 31: Visualizing Friction


College-Prep Physics: Here’s a great demo (stolen from Preconceptions in Mechanics) for visualizing the “interlocking bumps” model for sliding friction. Try pulling/pushing two wire brushes past each other (I actually have hairbrushes pictured).

  • Ask about which direction the bristles push on each other to help students see the directions of the frictional forces acting on each of the objects.
  • Use it to discuss how “smart” static friction is by “knowing” just how hard to pull to keep an object at rest: the bristles bend more/less — similar to our previous discussion of “smart” normal forces.
  • With 2 identical brushes it’s easy to discuss the equality of the friction forces. Swap out one brush for another with bristles of different material (wire, hair, plastic, etc.) and ask about equality again — similar to our previous discussion of balanced forces produced by springs/rubber bands of different stiffness.
  • The effect of normal force on friction: The larger the normal force between two surfaces, the more those surfaces are compressed together, and the more the bumps/bristles interlock making it harder to pass through each other. You can noticeably see and feel the difference using the brushes.
  • If you want students to have their own brush models to play with, PiM suggests giving a pair of inexpensive toothbrushes to each group of students.


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About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

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