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Bridging analogies

October 20, 2012

I finally got around to reading Clement’s article on bridging analogies,

Clement (1993), Using bridging analogies and anchoring intuitions to deal with students’ preconceptions in physics, J. Res. Sci. Teach. 30(10), 1241.

The article talks about using a sequence of analogies to help students connect an anchoring intuition (i.e. a preconception that agrees with the physicist’s view) to a new situation about which the student is unsure or for which the student holds a preconception that differs from the physicist’s view. Consider a book sitting on a table. Many students have difficulty understanding how the table is capable of exerting an upward force on the book (Clement refers to this as the target scenario). Now consider a book sitting on a spring. Many students will naturally say that the compressed spring exerts an upward force on the book (Clement refers to this as the anchoring intuition). It is common practice for instructors to make an analogy between the table and the spring as a way of explaining the upward force exerted by the table. The problem, Clement notes, is that many students find this analogy to be faulty. Students see the table and the spring as fundamentally different. In this case, the analogy fails to provide any explanation or mechanism for the force from the table force.

Clement’s proposal is to expand the analogy into several steps. Clement inserts “bridging” scenarios between the anchor and target, making use of multiple analogies to move from the anchor to the target. Clement also suggests ending with a (often microscopic) model that provides students with a causal mechanism and also helps justify the final analogy. The very last step in Clement’s approach is an experiment or demonstration to help validate the analogy. Here’s is Clement’s sequence for the book on the table.

Clement used pre- and post-tests in high school classes that did and did not make use of bridging analogies. Comparing pre-post gains for the topics of normal force, Newton’s 3rd law, and friction Clement found that the classes utilizing bridging analogies produced gains two to three times that of the control classes.

My class is just finishing up our unit on gravity and a student came to office hours this week asking how a table is able to exert an upward force. Given that I had just read Clement’s paper I thought I would try a bridging analogy. I started by asking the student to imagine a thin piece of wood rather than a sturdy desk and asked what would happen if we placed something heavy on top of the wood. The student recognized that the wood would bend some. I then jumped suggested that this is bending is similar to the compression of a spring. I gestured as if I was compressing a spring and said that we know the spring pushes back on my hand because I can feel it. I then said that the table is like a very very stiff spring. The book pushes on the table causing the table to compress ever so slightly. Just like a spring, the table doesn’t want to be compressed and so it pushes back on the book.

The student seemed satisfied with the explanation and said “I think that makes sense.” but I recognize that I rushed the discussion. I (think) it would have been better if after the bending board scenario if I had asked the student what else we’ve talked about that bends or compresses rather than brining up the spring myself. I also should have asked the student if the spring exerts a force rather than stating that idea myself. Still, I was amused to have such a clear opportunity to try something I read about in a paper and I think it was generally successful. I need to try this with more students and try to probe their understanding at each step along the way to try to really understand what students gain from step in Clement’s sequence.

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