College-Prep Physics: Started off with this Do Now.
Which went just fine….
So then I threw this at them. (Remember, we haven’t done dynamic cases yet.) And here’s how they voted:
Not surprising. So then I follow up with a demo, but ask them to predict first. And here’s how they voted:
What?!? I’m happy, but confused. And when several kids see my expression, a few kids say, “Wait. That’s just like the previous situation, isn’t it?”
So we test it out. Instead of roller skates, I have one kid sit on a cart. I set up a (spring scale)–(string)–(spring)–(string)–(spring scale) between the two people. Having a spring in the middle is nice because it allows the tension in the string to slowly change as the kid on the cart speeds up. Then I send the kid on the cart back and the spring/strings slow the kid down gradually. And the WHOLE time, the scales read the same. And it makes sense to them now, because they think/visualize the rope as one long spring than stretches and maintains even tension throughout. And when one person pulls harder, the whole string/pulls harder. The spring stretch/compresses evenly along its length. We don’t see one end of the spring stretched out more than the other end.
So then if the tension between the two people is always the same, who wins at tug of war? Why did the kid on the cart slide, but the kid standing on the floor didn’t slide? (Friction!)
So then we modify our diagrams from before.
And then we watch a few tug of war movies I found from another physics teacher who posted them on his class website, paying close attention to friction (or lack thereof).
NGSS Science and Engineering Practice #2: Developing and Using Models