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Collateral Damage

September 10, 2011

One of the worst things we do to students is try to convince them (through any means necessary) that objects fall at the same rate. I think we do this to them in the name of stamping out misconceptions, but I believe it is high up on the damaging things we do to introductory physics students. I believe some of us do it because we think it really is cool, but mostly in doing so, we steal the possibility of it ever being cool for students.

I think it is damaging for a couple of reasons:

We mascarade around with one or perhaps a few demonstrations, and propagate the myth that single (or few) experimental outcomes determine truth.

We attach this observation or decree to the value of “g”, long before students have a chance to even understand what acceleration means.

We tell the lie that mass doesn’t matter for falling objects, when it fact it does, doubly so. It influences both the gravitational force and the intertial response of the mass to net force. It is the interesting intersection of these two truths (along with certain approximations) that something like mass doesn’t matter emerges. [Aside: The canceling of m’s across an equal sign is an injustice to the grounding and coordination of ideas that’s really involved.]

We fail to let them in on the interesting and perplexing conundrum of how it might even be possible for objects of different masses to always move the same way. The conundrum of “Man, I know it’s harder to get the more massive objects moving. So how does the gravitational force “know” to pull harder on the more massive one and to pull less hard on the lighter one? Or I know that the more massive object is being pulled down harder, shouldn’t it fall harder too?” That is where I want my students to be… in it. In the conundrum, misconception or not.

We rig contraptions to prove our point about freefall in a vacuum, long before our students are even poised to understand how that contraption could possibly work and before they’ve had a chance to think about why one would even care to do physics in vacuum. Isn’t there enough physics around us without vacuums?

I believe we kill the patient to cure them of an ailment they never knew they had.

11 Comments leave one →
  1. September 10, 2011 11:24 am

    One way I try to get students into a conundrum is to ask “Why might a smart person say…” and present both sides. In this case, “Why might a smart person say heavier objects should fall faster?” and “Why might a smart person say heavier objects should fall slower?”

    I did this yesterday when talking about circular motion of two objects on a record player. “WMASPS the two objects have the same speed? Different speeds?”

    Of course, now thinking about Mindset, I question my use of “smart person.” Any alternatives?

    • September 10, 2011 11:56 am

      Frank, I think that’s a great thing to do. I do it all the time. I often just ask, what might someone else think. I find myself at times using phrases like “a reasonable person” or “smart person”, but I think you are right to be a little wary of the smart word. I also often use the phrase, “I want to hear all the ideas. What else could someone think? Why might they think that?”

      My slightly more ideal situation would be spending time talking about how to get two objects (a heavy one and a light one) across a a frozen pond, such that they move in the same way the entire time. We’d talk about it. We’d take some data. We’d talk about is some more. We’d make a challenge out it: which team can best pull four differently weighted carts across the room so there motion is identical? Then, later, maybe much later, we’d start studying falling object. We’d collect data. And I’d wait. I’d wait until someone sprung either the idea or the conundrum: “Wait a minute! all the masses are falling the same way? Isn’t this like what we did before? Does that mean… Wait, I though that the … How does…” What I hope to do is leverage a place where their intuition is right (heavy things are hard to move), and to extend that idea to how to get heavy and light things to identically move. Lastly, I want to help that intuition go in reverse. Instead of, “How would you make them move the same?”… “What might it mean when two objects move the same? What does that tell us about how gravity works?”

      • September 10, 2011 12:13 pm

        1. I’m going with “thoughtful person” from now on. I want students to discuss logic and reasoning behind possible outcomes, not just guessing.
        2. Do you have two or three carts? You link them together to change mass and turn fans on/off to vary force. Put in front of motion detector. Take data fire one cart/one fan, two carts/two fans, three carts/three fans. Very powerful example that we keep returning to.

      • September 11, 2011 3:28 pm

        I try to avoid qualifiers like thoughtful, reasonable or smart in these situations. I just ask them why a person might think or argue a certain point. I let them be the judges for if it is reasonable or thoughtful.

  2. September 10, 2011 2:31 pm

    @ Frank. I think there are variety of different ways you could get students to investigate it, depending on equipment, the students, and the time available.

    This conversation has gotten me thinking again about why we should try to think about g in the units of N /kg.

    It is also got me thinking about a series of “What ifs”? What if the force due to gravity near the earth’s surface was Fg = 9.8 N instead of Fg = # kg * 9.8 N/kg. What if the force was inversely proportional to mass? What would motion be like then?

    Lastly, this conversation has gotten me thinking about the strange coincidence that inertial mass and gravitational mass are equivalent, as far as we can tell.

    Conundrums indeed.

  3. September 10, 2011 3:33 pm

    Is your thinking about this inspired by the American Radio Works podcast about PER?

    • September 10, 2011 4:53 pm

      No, I actually haven’t had a chance to listen to it yet. I keep meaning to. It was actually in response to some intermediate-level physics majors trying to work through why all things fall the same before class–one of the ideas floating around was that they only seem the same because the masses are so little compared to the mass of the earth. I think they were thinking about universal law of gravitation–that since M_earth was huge, any variation in m_object would be so little that the effective “force” is about the same, and thus the acceleration was the same. They, of course, weren’t that explicit about their thinking. Then later that night, I was talking with a colleague about all the ideas (including misconceptions) that come up when you ask people to tinker and talk about falling objects.

    • September 11, 2011 5:01 pm

      OK I listened to it. I can see why you’d ask. It’s the specific question Joe reads from the FCI. I can’t imagine students being able to answer that correctly is a good proxy for understanding anything about Newton’s laws or empirical models for gravitation forces or their relationship.


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