The big three: inertia, velocity and acceleration
We have just finished our first five weeks of school, and my Physics First students have had their first run-ins with three of the most basic, yet most confusing concepts in physics: inertia, velocity and acceleration. After 23 years of teaching the subject, I have come to realize that I need to spend quite a bit of time trying to solidify these concepts in students’ minds.
Blame those stinking preconceptions, or the obtuse explanations in physics texts, but it is just really hard to get students to grasp those three concepts. Sure, they can memorize the definitions, but few really understand what they mean. Without a decent comprehension of them, learning later concepts (like force and momentum) is appreciably harder.
One misconception about inertia is that it is a force. That is, to some students, inertia is a force that keeps you at rest. A passable first definition, but then these students fail to realize that inertia also keeps you going. When the idea of force comes a bit later in the course, then they confuse inertia with real forces like gravitation and friction.
Inertia is a property of matter. It is internal, not external. External forces support an object, or resist other external forces, or push or pull an object. Inertia cannot make an object move by itself; it only maintains the motion that the object already has. If the object is at rest, it “wants” to stay at rest. If it’s moving in some direction at some speed, it “wants” to keep that speed and heading.
Simple ideas, to a physicist, but to a beginner, not so obvious.
I once read somewhere that most people have a basic Aristotelian idea of motion. Like the ancient Greek, they believe (without much critical analysis) that continuous motion requires continuous force. Aristotle taught that objects have to be pushed along in their sideways motion to maintain that motion. When the force runs out, the objects stop.
Thus, when I ask my favorite Newton’s First Law question, many students get it wrong. Here it is: “Four forces act on an airplane in flight, gravity (weight), lift, thrust and drag. What happens to the airplane when thrust = drag and weight = lift?”
Aficionados of the Law of Inertia know that when the net force on an object is zero, the objects maintains whatever motion it had at that point in time. Or to put it in physics shorthand, it has a constant velocity.
Many students, however, answer that (a) the airplane hovers or (b) it plummets to the ground, because, like Aristotle, they believe you need a non-zero force to keep the plane in the air. If that were true, we would not be flying in airplanes much now. Likewise, it takes a bit of coaching to get them to realize that spacecraft (science fiction films notwithstanding) do not need to constantly operate their propulsion systems to keep moving.
We need a Wii or XBox version of Asteroids, and make kids play it from the time they are kindergarten.
Tied up with the problem of inertia is the whole issue of velocity. We physics types know velocity includes direction, as well as speed, but getting students to remember that direction is integral to velocity is a hard nut to crack. Common usage conflates speed and velocity, so the distinction in physics has to be learned. And without understanding velocity’s directional nature (it’s a vector, Victor), understanding the Triune nature of acceleration is really difficult.
Constant velocity means constant speed and constant direction. The law of inertia says if the net force on an object is zero, the object will have a constant velocity. So objects at rest (velocity = zero) stay at rest. Objects in motion (velocity = something other than zero) keep on doing the same thing.
Until an external force changes the net force to something other than zero. Then the object’s velocity changes. (This is basically what Newton’s Second Law says, by the way.) There are at least three possible changes: speed up, slow down, or change direction –. the Triune nature of acceleration. Acceleration is a change in velocity, and that airplane is not going to anything different until the pilot (or a force of nature) changes one of those four forces acting on the plane.
Understand inertia and acceleration thoroughly, and celestial mechanics and centripetal forces (don’t get me started with centrifugal forces — that comes in a few weeks) fall into place. It takes some effort, but I think I eventually get the points across to them.
I tell my students and their anxious parents that learning physics is like learning a new language. Without the basics, learning new material is incredibly hard. So we need to drill on the basic stuff, like inertia, velocity and acceleration, so that learning more complex material will be actually easier than it would seem. (Well, except maybe for quantum mechanics!) As with a new language, no one should expect a student to be fluent after just one year’s instruction. So we cannot expect a student after a year of physics (or chem or bio) to be experts in those fields. We just want them to be past the Berlitz phrase book phase of learning.
October 1st, 2007 at 1:03 am
speaking of quantum physics, what do you make of the hypothesis that consciousness (through the medium of “observation”) is responsible for the reduction of quantum possibilities into the single observed outcome, as in the copenhagen interpreation or the collapse of the wavefunction to an eigenstate in the Zeno effect? i know such phenomena have been characterized as being caused by the interaction of a photon with the observed wavefunction during measurement, which forces it to “choose” a pure eigenstate. but do you think it’s possible that conscious observation, or the mind, is the real causal agent here? i know such an interpretation plays a role in john wheeler’s participatory universe and in chris langan’s cognitive-theoretic model of the universe.
eric g
October 1st, 2007 at 11:40 am
Attempting to measure something reduces the possibilities, but I would hesitate to say consciousness does. If that were true, merely thinking about measuring or observing an event would have the same effect as actually doing it.
The New Age and human potential movements, led by such luminaries as Deepok(ets) Chopra and characterized by that annoying movie, “What the Bleep Do We Know?”, suggest otherwise, of course. They expand the influence of measurement on quantum events to propose that mere thought influences macroscopic events. (I simplify, of course.) It’s a bogus argument, and a misuse of physics to support psychological effects.