Day 73 – Unbalanced Forces


Physics 11 – Students were applying their understanding of force diagrams, Fnet and unbalanced forces to problems today.  I usually don’t get too prescriptive on problem solving techniques, but I spent three days emphasizing #1 and #2 in the photo. As all teachers know, kids really resist doing things they think are not necessary. Unfortunately for them, skipping steps 1 and 2 will almost certainly result in mistakes even for the smartest kids with the most intuitive understanding of physics.

The Big Tip for Physics Teachers

Ok, so we all know that kids try to skip steps. My students would often mix up writing out an equation for the unbalanced forces (Fnet equation, Fnet = ….) with Newton’s 2nd Law (Fnet = ma).  Here is the new thing I found out this year: after kids draw and label their FBD, they become paralyzed on the next step because they realize they don’t know what all the forces are.

I used to think that they don’t write out Fnet = .. + … + … because they were trying to skip a step. What’s really happening is that when they start to write out Fnet = …. what they’re expecting is that they should already know everything on the right hand side of the equation.  They often don’t, and this becomes a discrepant event for them.

There is a strong link between this and a phenomenon in math.  In math, students often have a difficult time understanding the “=” in an equation.  Many people don’t see equations as being a statement of equivalence.  Instead, they see an equation as a question on the LHS (what is?) and the answer on the RHS. This is what happens with Fnet. The intuition is to have an answer for everything on the RHS.  Note: just because students can give a definition of what the equals sign means, doesn’t mean they treat it like that when working with math or physics.


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