Day 71: Modeling is So Worth It!Posted: December 18, 2012
Today we rolled balls off a table. I had intro’d the plan yesterday by asking students about how we can investigate an object flying through the air. We talked about how motion detectors are fairly useless, but they remembered way back when (mid September, I would guess) when we did a bit of video analysis for CV and CA before switching to motion detectors (I start CV with the CV buggies with penny drops to mark postions at equal time intervals, so I liked using video analysis right after that because the clicks are essentially penny drops). They figured video should work well.
I had them start clicking before the ball actually left the table. This made it very evident that the X position graph stayed linear with the same slope and the X velocity graph stayed constant at the same values. They picked up on this right away in the whiteboarding circles. It didn’t take them too long to figure out that the Y direction can be modeled with CA and that the acceleration should be 9.8 m/s/s.
The best part was when they stated X velocity was constant and I asked “Why?” (and probably made a pun about Y at the same time…). Because there are no forces in the X direction. You mean the forces are balanced in the X direction? Yep. They were able to use the same logic at that point show why theory says that the acceleration in the Y direction would be 9.8 m/s/s based on the free body diagram. Again, I love connections!
Some action shots for fun.
A student before rolling the ball
While the ball flies (see the blur?)
Students analyzing the data