Constant Velocity & Forces

This is just a quick post to get down my thoughts, but I’d love to get some feedback.

So I tried to teach with a blend of the physic by inquiry and modeling instruction that I understand.  It worked okay, but there were a few things that didn’t go super smoothly.  One of those was definitely getting stuck in the kinematics doldrums.

I even tried to introduce balanced forces before moving on to acceleration (as suggested by Kelly O’Shea), but I couldn’t find a experiment that made the idea stick.  Instead I reverted back to unbalanced forces cause acceleration – therefore no net force means no acceleration.  At the end of the year, a solid chunk of my students had reverted back to thinking a net force is required in order to move an object at all.  Ugh.

So for some reason I was just thinking about this, and I think I have an experiment/situation that might enable me to introduce balanced forces before acceleration.  Here’s the idea:

Make a train with the following sections:  Constant Velocity car, spring force scale/gauge, dynamics cart, another spring scale, and a friction block.  Have the students determine if the cart in the middle matches the constant velocity particle model.  Record the measurements on the force gauges.  Try a different friction block (to get different forces, but small enough the car can still pull), check for constant velocity and record the forces.

I think that would flow nicely to adding that part of the constant velocity model is balanced forces.

I haven’t done any modeling workshops, so I’m really just picking up the parts I understand.  Is there anything else I’m missing here?  Do you think this will work?  Do you think this won’t work?  Constructive criticism needed!