In science, you can’t always get what you want

I teach one Introductory course in elementary school physics majors, but my classes aren’t really about physics. It may look like this at first glance, but it’s a trick. The course explores the nature of science. That is what makes it so great.

When I talk about the nature of science, I’m not referring to the list of steps outlined on this poster in your fourth grade classroom – that’s not how science works. Instead, think of the nature of science as both the process and the limits of scientific endeavor. Let me explain using an example from my class.

Constant force movement

The activity begins with a seemingly simple question: what happens when you push a low-friction cart with a constant force force? Before collecting evidence, students often say that the car accelerates for some time and then reaches a constant speed. This is because most of them equate constant force with constant speed. In fact, this is true for most people, not just primary education majors.

Students can easily test this idea with a simple experiment. A small fan attached to a cart can exert a constant force, and a motion detector provides a graph of speed versus time. You will get a graphic that looks like this:

This clearly shows that the speed of the car increases over time without reaching a constant speed. But wait, the students say, this is a short distance of just over a meter. What if you use a longer track? OK, I can do that too:

Fan Cart 1

Yes, the car is still accelerating all the time. If you want to do homework, use video analytics to plot speed and time. But now to the real question: What happens on a super long route? Will the car accelerate all the time?

Students offer all types of answers, usually as follows:

“I thought the car was moving at a constant speed. Based on the graph we made, it seems like it just keeps accelerating. If you have a longer track, the car will keep accelerating.”

“I agree that our data shows that the car gets faster over the short distance, but I don’t think it will accelerate any further. That doesn’t seem realistic. Otherwise it would reach a ridiculous speed.”

But that’s the real question: would the car keep accelerating forever? With this question, the subject shifts from the force to the nature of science.

The experimental nature of science

What I like best about my fan cart experiment is that it isn’t perfect. Clearly, if I want students to explore the movement of the fan cart, they can use a longer track. You certainly see the problems in that. What if you double the length of the route and the car just keeps going faster? Does that mean it will pick up speed forever? It is clear that simply building a longer track doesn’t answer the question. Second, a longer route requires more money, space, and time. Oh sure, in this case the price remains negligible. But it proves my point: you can’t always do a longer track just because you want one.

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