Dec 22, 2010

Newton's 2nd Law Survey

I got myself into some trouble yesterday while out to breakfast with a few friends. Our conversation was perfectly civil until I brought up an idea for an informal research project I'd been curious about taking on to reveal common misconceptions about Newton's 2nd law. I'd been imagining walking the streets of New York City with a microphone and performing an informal survey of how people answer a question such as, "Why does an egg break against the floor when you drop it?"

My friends and I first discussed the logistics of such an operation: whether a person on the street is more or less likely to walk hastily away if the interviewer is wearing a lab coat and bow tie... or maybe even just holding a clipboard. We agreed that it would be important to record the audio, so I could go back and listen carefully to the wording of the answers.

I then gave my friends an opportunity to respond to the question in their own words, and this is where the trouble/fun began. I got a variety of answers, such as "the force of gravity collides with the force of the floor and the egg gets caught in the middle." I expanded the question to include an egg breaking when thrown against a horizontal wall, and these answers included "the egg is too brittle withstand the force of your throw when that force encounters the resistance of wall." I, unfortunately, did not record the audio, so I can't give you the precise responses, but the most relevant point here is that I felt that their answers reflected some common misconceptions about Newton's 2nd law.

However, when I revealed to my friends that they'd answered the question incorrectly, they all got very defensive. They felt that a) I had tried to trap them into giving incorrect answers by asking an unclear question,
b) I had tempted them with incorrect reasoning by paraphrasing answers I knew were incorrect ("It sounds like you're saying..."), and c) I was nitpicking about the precise definitions of words like "force" and "inertia," rather than focusing on the conceptual validity of a response.

Their first two criticisms were certainly valid. My friends said afterward that they had been focusing on what aspects of the structure of the egg shell cause it to break, in contrast to "a steel egg." They felt that my clarifying questions interfered with their reasoning process unfairly, and that my facial expressions and tone of voice helped to convince them that these incorrect explanations were in fact correct. When they finally learned that this explanation was incorrect, they felt betrayed and angry. One friend remarked that treating a person on the street in this way might get me punched in the face.

The third criticism, however, is much more complicated. When I ask this question, I am most interested in whether the conceptual framework of the answer is correct: whether the individual is invoking the spirit of Newton's laws in their answer. However, I believe strongly that some mistakes in terminology do reflect actual flaws in understanding. If someone doesn't use the word "force" at all (the word push is equally valid in this context), I would certainly be satisfied with answer that was conceptually correct: The table had to push against the egg really hard to stop it quickly, and eggs break when they get pushed that hard. But is it possible for a conceptually valid answer to misuse the word "force" to mean inertia? I would say no, that if someone is using that same word "force" to describe the interaction between the floor and the egg, then this reflects true misunderstanding of the force concept.

If I'm actually going to go out on the street and ask people this question, I might need to change a few things:

• The question needs revision. For starters, an egg doesn't break when you drop it, it breaks with it hits the floor! Perhaps this prompt would be better: Imagine you drop a fragile object on the floor and it breaks. Describe the interactions between any objects involved that result in the breaking of the object.

• I'd need to shut my mouth and let people explain to their own satisfaction with no interference on my part. I imagine this is common practice for researchers collecting data in this way.

• I could ask for an explanation that did not use some words (force, acceleration, mass), so as to prevent confusions arising from confusions with these terms.

• I could ask a series of smaller, simpler questions to hone in more closely on where the misconceptions lay. This is the approach of a multiple choice diagnostic like the FCI, but this approach runs a risk of categorizing people's misconceptions too early in the process.

If I was able to collect adequate data, I could examine the relevance of each of these factors. Since I often use this question as an example of the validity of a conceptual approach to physics education, this would be worth pursuing. I'll keep you posted on my progress!

2 Responses so far

  1. Anonymous says:

    The breaking of an egg has little to do with the force or forces on it. The total force on an egg can sum to zero and yet it shatters (e.g. an egg in a vice) or the total force can be enormous (egg in freefall near a massive star) and the egg not shatter. Nor do large forces in opposing directions do the trick - consider gravitational forces by different massive bodies on the egg.

    I think a full correct answer is perhaps more subtle than you intend.

  2. JK says:

    This comment brings up some important distinctions, and I absolutely agree that the truly accurate answer is more subtle than I intend here. I disagree, however, that the breaking of an egg has little to do with forces. There are some subtleties that would be welcome in even a casual discussion of Newton's 2nd Law.

    We are certainly NOT concerned with the "total force" or net force, as illustrated by the example of the egg in a vice. Though it's true that an egg in freefall near a massive star would be left intact, an egg falling near a black hole would certainly break.

    Since an egg is not a point particle, we must analyze the difference in force applied to different particles in the eggshell, and the molecular forces between particles that normally hold the egg together. Subtle indeed!

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