In the picture you see a 200 g mass floating inside a very light cup. The water is inside a large beaker. The water level is marked on the side of the beaker.
I'm going to remove the 200 g mass from the cup and place it back in the water on the bottom of the beaker. How does the new water level compare to the marked level? Give your reasoning in the comments.
In the most recent post, I explained about arguments and the new AP Physics 1 and 2 courses. We absolutely must get our students discussing physics, arguing about physics, making errors and catching errors. I personally think in terms of "physics fights", the ritual debates over research-style problems that underlie the US Invitational Young Physicists Tournament.
In order to argue about physics, first we've gotta have something specific to argue about. We need problems that are simple enough to be accessible to first-year physics students, problems that are within the scope of AP Physics 1 and 2; but also, these problems must be complex enough to, well, produce reasonable and legitimate disagreement about physics. Arguments can't be artificial. If you present a ridiculously bogus line of reasoning to your class, they not only won't engage, they won't even buy in to the necessary process of discussion.
My primary piece of advice is to be flexible in your teaching. When a problem organically provokes a good discussion, go with it! Engage that authentic argument until it's resolved, even if you hadn't anticipated it. Similarly, if a problem that you expected to be tough gets the whole class nodding their heads in agreement, just move on.
Last week at my Mahopac, NY institute, the problem above provoked an unplanned but long and deep discussion. I heard four different lines of reasoning from the participating physics teachers. Five minutes of talking amongst themselves failed to resolve anything. So we kept talking. It was tough for some of the teachers to articulate their reasoning; others articulated clearly, but didn't convince their colleagues. I'm an old debate coach... I noticed how a bunch of teachers knew that an argument was incorrect, but couldn't address precisely why it was incorrect; all they could do was reiterate their own argument. (In debate we call that a failure to "clash.")
Of course, what I love about physics over debate is that once the discussion petered out, we just did the experiment. Nature is the ultimate judge, not nine political appointees.
So what's your answer? Please post a comment with your line of reasoning. Don't be afraid to be wrong -- all Jacobs Physics readers are teammates, we all love each other like brothers and sisters. The only justification I don't want -- for now -- is "I did the experiment, and this is what happened." Comments are moderated so we don't get linkspam, so it'll take a few hours for me to post them.