Day 15: Normal Force Conundrum
Student A: “The forces are ALWAYS EQUAL. That’s what we’ve been talking about for the past week.”
Student B: “Well, if B is at rest, then the two forces must be equal.”
Student C: “The force from C must be greater since it bears more weight.”
Student B: “But if the force from C is greater, then B would move upwards.”
We’re in a difficult spot right now, and I probably should have delayed this question until after we talked about gravitational forces in more detail. But, it was on the HW sheet for a Preconception in Mechanics lesson that we already did, so I just photocopied the worksheet without really looking at it first. *hangs head in shame*
We were able to get to a point where:
- We understand that the force from C must be greater because the surfaces between B & C deforms more than the surfaces between A & B, since surface C is supporting the weight of both block.
- We don’t understand why block B remains at rest, since the 2 forces on B are different in size.
My best answer to them was to put a big question mark on it and return to it later. We don’t have all the pieces to this puzzle yet. Students didn’t really like that answer and left class a bit frustrated.
I really resisted doing an impromptu gravitational force lesson, since I’m saving it for later because that’s how Preconceptions in Mechanics does it. I think the reason they delay gravitational forces and do normal forces first is because kids often confuse the two forces when objects are resting on top of each other. For example, using the problem above, many kids think that the gravitational force on A literally is the normal force of A pushing down on B. In this case, they have the same magnitude, but they are really two distinct forces.