Teach Fzx Tuesday - Dynamic Equilibrium

Following a test on static equilibrium we move on to what I like to call "dynamic equilibrium."  There is no real difference between them but my students think there is.

As an introduction, they sketch the side view of their favorite vehicle on a straight, flat road moving to the right at a constant velocity of 78 mph (or whatever speed they give me).

Then I ask them to sketch a Free Body Diagram of the vehicle.  We talk about what to name the forces but I am careful to emphasize only that the vehicle is moving at a constant speed on a level, straight road.

Here's the fun part - I ask them to clearly indicate which force is larger for the vertical pair and then for horizontal pair and here's what I see on 95% of student FBDs:

They understand that the up and down forces balance, but seem to think that the horizontal must be unbalanced forward even for constant velocity.  Then I ask

Q - "If you are at a constant speed in a straight line and the forward force is bigger than the backward force, what happens to your speed?"
          A - "You speed up"

Q - "If you are at a constant speed in a straight line and the backward force is bigger than the forward force, what happens to your speed?"
           A - "You slow down"

Those same students who are so universally wrong on the FBD can answer the questions correctly. Interesting.  It seems that we tend to believe that any object in motion needs a net force forward to remain in motion.  When Galileo realized that moving objects naturally tend to keep moving at a constant velocity (inertia) it was a triumph of science.  ZERO NET FORCE means ZERO CHANGE.

The first and second CONDITIONS FOR EQUILIBRIUM are not "conditions for static equilibrium."  They apply to all objects that are at a constant velocity since "rest" is an illusion - all speeds are measured relative to something else.  Forces and torques balance for both "static equilibrium" and "dynamic equilibrium."

So it's a Fzx teacher's job to take our students' Aristotelian misconceptions and move their minds forward to Galileo and Newton - then they're only three centuries behind.