Day 33: Why Doesn’t the Book Fall?
College-Prep Physics: As you can see, I had a number of groups struggle with drawing the free-body diagram for the question given. Namely, most groups left out the upwards force of friction.
ME: “Based solely on your force diagram, what would happen to the book?”
S: “It would fall.”
ME: “But it doesn’t. Why?”
They thought that just squeezing the book horizontally between the log and wall would keep it from falling. So then we did a few things:
1. We simulated the scenario using our hairbrushes. The two wooden brushes rubberbanded together represent the book, while the two black plastic brushes on each side represent the log and the wall. So what prevents the wooden brushes from falling? (Friction!) Which direction must it act? (Up!)
2. We simulated the scenario without friction. The whiteboard in the middle represents the book. The carts on each side represent the log and the wall. I held on the whiteboard in the air, while I had one of the students press the 2 carts together as tightly as they could in order to try to hold up the whiteboard. As soon as I let go of the whiteboard, it fell to the floor, no matter how hard the carts were pushed against the whiteboard. (I think this demo is more easily understood than the frictionless wall demo I did previously.)