Day 17: Force Discussion to FBD Practice

Because of some interruptions to our class time, it took us a while to (finally) get to the end of problem 4. It’s a tough one—asking what will happen, after you’ve been pushing an object at a constant velocity, when you then reduce your push to use a smaller force.

The class was divided among a lot of different ideas. Would it go at a slower constant speed? But were the forces still balanced? Would it speed up, since they thought we had seen that any constant force would cause an object to keep speeding up? Would it slow down?

At some point, we decided to try it out with a spring scale. It was tough to do that carefully, though, and some students suggested that we could simulate the situation with the two fan carts (one on high—friction; one on low—normal force).


I don’t have a photo of the fan carts, but it worked out pretty well. No matter what, that setup of the fan carts slowed down (when it started out already moving). They also started speeding back up in the other direction, but everyone agreed that wasn’t going to happen with friction and the box on the ground.

After we finished that discussion, we jumped right into working on the two pages of practice with drawing a variety of FBDs. They worked in groups, I encouraged them to go back to the front of their packets and remind themselves about what we’d decided for directions of forces, and they argued a bit among themselves. We ran out of time to whiteboard, but we will start there right away in the next class.

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