After years of serving as a physics teaching assistant at several public American universities, I’ve come to an alarming conclusion: students in today’s general physics courses (i.e. courses that don’t require calculus, intended for non-physicists) aren’t being taught physics. They might be learning how to mechanically calculate answers, but they’re learning very little about the actual scientific process of inquiring about the nature of reality. They aren’t learning how these physical laws were deduced in the first place– which is far more important than the intricate details of those laws. I think this educational deficiency contributes to widespread misconceptions such as the belief that the Earth’s seasons are caused by variations in the distance from the Sun, and the curious notion that toilets flush in opposite directions on different sides of the equator. Instead of teaching students how to crunch through equations and obtain the “correct answer,” physics courses for general education need to place a higher emphasis on other aspects of the scientific method. Namely, students need to be taught how to evaluate a belief for validity using observations and logic.
I’d describe my application of the scientific method like this: sometimes I notice something strange in my environment, or take a fresh look at a ubiquitous phenomenon. My curiosity’s piqued, so I make at least a token attempt to explain the phenomenon. Historically, the most effective way to approach these problems has been to assume that the universe is governed by objective rules and try to guess a rule that explains the phenomenon in question without implying absurdities. In physics, this search for a mechanism generally involves postulating a general principle (for instance, “energy is conserved” or “objects at rest stay at rest”) and considering the consequences of that postulate. The resulting explanation is “scientific” if it makes predictions that can be verified. This is the primary characteristic of “good” science; not only do our models of reality have to be consistent with the facts at hand, they also have to predict new phenomena which can then be checked via experiment.
Do general physics classes in their current form actually teach students to think using this process? I don’t think so, mainly due to the focus of the course material. A typical problem in one of these classes is formulated like this: “Given this verified physical law, predict the behavior of the following system.” As an example, students are often told to assume Newton’s laws of motion and predict the path of a baseball after it’s thrown. This type of exercise tends to leave the student with the impression that science is a mechanical system of picking a set of equations and churning out an answer. Problems like these are certainly good practice for learning how to apply physical laws to obtain specific predictions, but that’s nearly irrelevant for a student who doesn’t intend to become a physicist.
A better approach would actually require the students to construct hypotheses regarding phenomena that seem confusing, and have them test these hypotheses by considering what they imply about the real world. Though we’re probably succeeding at teaching people the mechanics of working within a given scientific theory, it’s more important to show them how those theories were constructed in the first place. Students need to be shown how to take an explanation of a phenomenon and test it for validity.
We’re failing miserably at this goal.
For instance, if college graduates were randomly surveyed regarding the cause of the Earth’s seasons, I think a sizable percentage of them would say that the seasons are caused by variations in the Earth’s distance away from the Sun. This misconception only arises because students aren’t being taught to critically evaluate ideas in terms of their implied effects. Consider this: if winter occurs when the Earth’s farther away from the Sun, doesn’t that idea imply that winter should occur at the same calendar date everywhere on the globe? If seasons really were caused by the Earth’s distance away from the Sun, the seasons would be the same everywhere because the Earth as a whole would be closer or farther from the Sun. This isn’t true, though: in the southern hemisphere, summer occurs in January, and winter occurs in June (vice versa for the northern hemisphere). In other words, if you believe that seasons are caused by distances from the Sun, you’d be likely to pack completely inappropriate clothes for a “summer” plane trip across the equator.
What’s the actual cause of the seasons, though? More importantly, how do you determine that cause? I’ll answer that question with another question: Why do winters get colder as you move farther away from the equator? Is it because the distance to the Sun increases as you approach the planet’s poles? No, not in any significant sense; the Earth’s usually ~150,000,000 kilometers away from the Sun and the Earth’s only ~12,000 kilometers in diameter. So any movement you make on the Earth’s surface is an infinitesimal fraction of the distance to the Sun; it shouldn’t play a role in determining temperature.
However, the Sun is lower in the sky when you’re farther from the equator. This relationship’s easiest to see if you draw a picture of the Earth in its orbit with the axis of rotation of the Earth (roughly) perpendicular to the line that connects the Earth and the Sun. A person standing on the equator will see the Sun pass nearly directly overhead during the course of the day, but a person very far north (or south) of the equator will never see the Sun directly overhead. This means that less light intensity is reaching the Earth’s surface at those points far from the equator- less light per square meter is hitting the ground because it’s not shining directly vertically. Again, the best way to see this is to draw a picture and investigate the geometry of the situation on your own.
How does this explain the seasons, though? The seasons occur because the axis of rotation of the Earth’s actually tilted away from being perpendicular to the line that connects the Sun and the Earth. This axis stays in the exact same orientation (relative to the “fixed stars”) as the Earth orbits the Sun, so sometimes the northern hemisphere’s pointed towards the Sun (this produces a northern summer and southern winter) and sometimes the southern hemisphere’s pointed towards the Sun (which occurs during northern winter and southern summer).
The actual scientific process involved in this explanation is the recognition that a common misconception (seasons being caused by distance changes) predicts incorrect phenomenon (winter and summer occurring at the same time everywhere on the globe). This realization’s followed by a different model for the seasons (the tilt of the axis) and should then be followed by observations that confirm that model. For instance, you could observe the Sun staying lower in the sky during winter than during summer to verify the “axis tilt” model of the seasons.
This brings me to my next pet peeve. In the Simpsons episode “Bart vs. Australia,” the U.S. Embassy in Australia has gotten fed up with the way water drains in the southern hemisphere and installed a special device on the toilet to “make the water drain in the American direction.” I believe this scene’s representative of a widespread belief that water drains counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.
The purported cause of this difference is a real phenomenon called the Coriolis effect. In order to understand the Coriolis effect, consider a person standing on the equator. Because of the rotation of the Earth, he’s actually moving in a large circle at a very high speed. A person sitting exactly on top of the North or South Pole, in contrast, would simply be spinning once every 24 hours- he wouldn’t be going anywhere. At intermediate latitudes the speed varies smoothly from a maximum value at the equator to zero at the poles.
Suppose you stand at the equator and fire a cannonball directly north. In addition to the northward velocity from the cannon, the cannonball has the high velocity of rotation of the equator, and as it flies north it retains that velocity. However, once the cannonball travels a significant distance, the ground underneath it isn’t moving as fast as the cannonball so its path will appear curved relative to the ground. Because the Earth rotates from west to east, the cannonball will appear to be “pushed” to the east by the Coriolis effect. This is the modern explanation for why hurricanes rotate in preferred directions in different hemispheres.
Does this explanation apply to toilet bowls and sinks, though? The cannonball’s path isn’t noticeably curved unless the cannonball travels very far to the north. That’s why hurricanes are affected by the Coriolis effect- they’re large enough so that the eastward deflection is sizable. But toilet bowls and sinks are millions of times smaller- the Coriolis effect’s way too small to have any measurable effect on them. The tiniest imperfection in the shape of a sink’s drain or random circulation of water flowing down it would completely mask any Coriolis-induced spinning.
That’s a physics explanation of why this “rotational drainage” myth isn’t true. I included it for completeness’ sake, but my main point is that physics education shouldn’t be about supplying explanations to students. Science education should teach students how to create and test their own explanations. Let’s say that I did believe that sinks drained in preferred directions. How would I test this belief for accuracy? Well, I’d just have to wash my hands in different sinks and see whether the water drained in a consistent direction in this hemisphere. Try it, and you’ll see that this simply doesn’t occur- water drains clockwise or counterclockwise in different sinks even when you’re far from the equator. Hint: if it’s difficult to see which way the water’s draining, try sprinkling some pepper on the water so you can see the circulation pattern more clearly.
Why do otherwise intelligent people believe in effects that can be disproved just by looking in the sink? I think it’s because no one ever takes the time to emphasize the importance of verifying beliefs through experiment. That’s what science classes should be doing, but instead they’re teaching medical students the arcane intricacies of Newtonian mechanics without even attempting to test whether or not those students understand how Newton discovered his theory in the first place.
It should now be obvious that there’s an inadequacy in our science education system. Why does this inadequacy exist, though? The obvious reason is that a mechanical, computational approach is easier to apply in larger classes, and it easily translates into an objective grading system. This is probably unavoidable in today’s large universities, but part of the problem’s due to the fact that general physics classes are treated as “watered-down” versions of the introductory physics courses that are intended for physics majors. This is convenient for the instructor, of course- the material’s easy to translate from one class to another. It’s inappropriate for the students, though, because the major-based physics courses are simply treated as the first stepping stone to building a scientific mindset. These first courses are where physics majors gain practice at mathematical computations; the real science actually comes into play much later. For general students, however, these few physics courses are the only glimpse of the subject they will ever see. We can’t afford to ignore the more important aspects of the scientific method, even if it means the class structure has to be changed significantly.
- I mention this possibility because many students suggest it as a possible reason why winters are colder when you’re farther from the equator. ↩ back
- The axis is actually 23.5° away from being perpendicular. ↩ back
- Due to the geometry of Earth’s orbit around the Sun, northern winters occur when the Earth is closest to the Sun. ↩ back
- A person on the equator is rotating on a circle of radius 6400 km (the radius of the Earth) and they complete one circle a day, so their speed is ~460 meters per second (greater than the speed of sound at standard temperature and pressure!). ↩ back
- Ignore air resistance for the purposes of this explanation. The cannonball simply obeys Newton’s First Law: “Objects in motion remain in motion.” ↩ back
(Ed. note: This article was originally written on 2003-08-20.)Last modified February 6th, 2012