Friction, a major grind
The other BFPM challenge problem was students to determine what forces the frictional force depends on. Students do the pretty typical experiment of pulling felt covered blocks along different surfaces, and come to the the conclusion that “weight affects the frictional force.”
“Really,” I ask, and push down on the block as the student is pulling it. “Did the weight of the block just change?”
“No” the student realizes, “it must be the normal force on the block that affects the frictional force.”
And in discussion, I marvel as kids figure out the difference between static and kinetic friction, realize how the static frictional force can vary depending on the other forces applied to the object. These are concepts that used to take days of me lecturing for kids to get, but in discussion, with a few questions, kids come to these conclusions all on their own.