One of the students in our physics teaching program has been organizing, “Physics at the Pub” events. Basically, a bunch of us get together to talk physics over beer and food. So far, we’ve met twice, and we’ve had some really fun, interesting conversations.
The first time, we talked about this question. “Why when you pour beer in the pint glass does it seem like the beer extends all the way to the very edge?” Like we know (and can see when the beer isn’t there) that the glass boundary has a definite thickness to it. So the beer must really be “inside” of the glass rim, but it looks like it extends all the way into the glass.
Last night, we started off by talking about the Veritasium’s bullet in the block question, but spent almost all the time talking about this situation: Why does a soccer ball go farther than a bowling ball when you kick it? Most people’s initial response is that “same kick” = “same force”, and thus by Newton’s 2nd law “Same force over more mass means less acceleration”, and then finally “less acceleration means less distance”
But, pretty quickly the discussion comes to be about what “same kick” means. It turns out, there are lots of reasons to be suspicious of the assumption that the “same kick” results in the same force, but it’s challenging work to reconcile that with the commonsense idea that you “kick with a force”.
What I love about this second question (which I’m pretty sure I was introduced to by David Hammer), is that, for students, resolving it involves having to really contend with many, many concepts all at once: what force is and isn’t, the limitations/challenges of Newton’s 2nd Law reasoning in the context of non-constant forces; their level of commitment to Newton’s 3rd Law, carving out how we should think about the magnitude of acceleration in concert with the duration of acceleration, impulse and momentum ideas, inertia and what we mean by that, and struggling with how t think about the “stiffness” of materials (and modeling that with springs). Energy typically gets in the mix as well.
The second thing I love about the question is this: Everyone knows the answer. The soccer ball goes farther. We just can’t agree on how to adequately explain it.
The third thing I love about the question is this: It reminds me that, if you want to find out what students think and/or engage them in deep physics, you don’t need an elaborate/ contrived scenario. In fact, the more everyday and seemingly simple the situation is, the more likely you are to engage their thinking. That said, this question should be probably saved for students who have a good deal of facility reasoning about Newton’s laws and who are likely to recognize the need to persist in trying to reconcile inconsistencies.