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The case against friction

February 16, 2013

In Hands-on-Science we spend about a week and a half talking about forces in “frictionless” environments before we formally bring friction into the picture. In the past I have tried to open up a class discussion as to whether we think friction is a force or something else. This discussion never went very far as almost everyone would very quickly state matter-of-factly that friction is a force. This semester I took a slightly different approach and asked every group to write down one argument in favor of classifying friction as a force and one argument against. The arguments in favor were pretty standard but the arguments against were very interesting and give good insight into 1) students’ interpretations of what forces should be able to do and 2) students’ understanding of friction.

Arguments in favor

  • Friction can cause changes in speed.
  • Friction is capable of performing a “push”*.
  • Friction can cause changes in energy.
  • Friction can slow an object down and it isn’t an energy so it must be a force.

Arguments against

  • Friction is not a clear “push” or “pull”.
  • Friction only acts in response to another force.
  • Friction only acts on moving objects.
  • You can’t control when friction acts on an object or the strength of friction like you can with a push.
  • Friction can’t make an object change directions.
  • Friction can’t be an energy source*.
  • Friction can’t increase an object’s speed.
  • Friction can’t initiate a change in speed.
  • Friction is so small that sometimes it can be ignored.
  • We learned that once the hand loses contact with the cart the force is gone.

The majority of arguments against calling friction a force are actually things that friction can do we just haven’t seen examples yet. This is good. It suggests that students have a pretty good grasp of the general force concept. These expectations for forces should be helpful next chapter when we’re debating whether non-contact interactions should be considered forces.

The arguments with asterisks treat friction as an object. This is a general concern throughout the course of differentiating between physical objects, forces, and forms of energy.

I can think of two interpretations of the last argument against. They might be saying that our earlier activities ignored friction and it can be confusing to add in new ideas later. Or they might be saying that all of the other forces we’ve considered so far were present when objects were touching and absent when the contact was lost but a box can be sitting still on the floor and not experiencing a force of friction. Unfortunately I didn’t have a chance to ask the group which interpretation they had in mind.


From → Teaching

  1. Andy "SuperFly" Rundquist permalink

    How would looking at friction microscopically help?

  2. That’s a good question. We usually talk very briefly about the microscopic model of friction but only in response to the “friction isn’t a push” sort of argument. I’ll have to think about how the microscopic model relates to some of the other arguments.

    • Andy "SuperFly" Rundquist permalink

      I wonder if the microscopic picture could help explain how it truly is a force just like any other, but, on the other hand, get the students frustrated with the complexity involved in ignoring the energy transfers to all of those molecules.

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