Several lessons learned from Randall Knight:
Students fail to learn mechanics from "physics 101" type classes for some identifiable reasons. For one thing, they come in with extensive phenomenological knowledge, but their understanding of it is extremely flawed. A student has ridden a bike at one time or another in his life, and he knows that to move twice as fast you pedal twice as hard. In his mind
.This is deeply wrong, and we tell students that forces actually accelerate objects. Unfortunately, old habits die hard. Consider the following setup: someone tosses a ball up in the air. On its upward arc, what are the forces on it? Before instruction, about 18% get this answer correct, that there is a force of gravity downward, and nothing else. The rest of students normally identify a "motive force" upward, keeping the object moving. After instruction, 25% get the same question correct.
In mechanics, an instructor will indicate that an object sitting motionless on a table is in equilibrium, meaning that since gravity was obviously pointing down, there must be a "normal force" holding it motionless. When asked, many students will respond that they don't actually believe in the normal force, but that they use it in the process of solving problems because it's obviously required. They haven't seen any mechanism, they don't see a velocity (which force causes) and they don't accept your premise about equilibrium.
Conveying electromagnetism is debatably even harder at the intro level, because students have very little knowledge of phenomena. They have changed batteries, plugged things in, and stuck magnets onto the fridge, but beyond that they don't know much. We don't stop to teach phenomenology, and instead strike right into a heavily mathematical discussion of fields. Moreover, the math often doesn't come back to the phenomena. The most common phenomena a person sees regarding electromagnetism is that of polarization, which explains why magnets stick to stuff, why a charged rod attracts stuff, etc. Students don't know about polarization, and often tend to equate positive magnetic fields with positive charge. They think that a magnet sticks because it has positive charge (north pole) and the fridge has negative charge. When asked what happens if you turn the magnet around, they sometimes reply that "that's why they put the sticker on one side."
I do think there is a limitation to how much physics most students will learn, but we certainly could do a lot better at the introductory level. Maybe then people would understand that magnetic bracelets, etc, don't do anything.
