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Well, in Mathematics there are somethings, which appear true but they aren't true. Naive students often get fooled by these results.

Let me consider a very simple example. As a child one learns this formula $$(a+b)^{2} =a^{2}+ 2 \cdot a \cdot b + b^{2}$$ But as one mature's he applies this same formula for Matrices. That is given any two $n \times n$ square matrices, one believes that this result is true: $$(A+B)^{2} = A^{2} + 2 \cdot A \cdot B +B^{2}$$ But eventually this is false as Matrices aren't necessarily commutative.

I would like to know whether there any such things happening with physics students as well. My motivation came from the following MO thread, which many of you might take a look into:

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Community wiki? –  Marek Nov 17 '10 at 23:10
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@MArek: i didnt find the option. if anyone can do it they are welcome –  Chandrasekhar Nov 17 '10 at 23:15
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@Chandru: AFAIK StackExchange recently changed its rules about this matter so that only moderators can make a question community wiki (the rationale being that the CW option is being misused at StackOverflow). –  Marek Nov 17 '10 at 23:26
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While i perceive the question interesting, I think the example with matrices is rather 'a common silly mistake' than 'a common false belief'. –  Piotr Migdal Nov 17 '10 at 23:48
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It seems a bit odd to be accepting a single answer to a soft question... –  David Z Dec 1 '10 at 0:21
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49 Answers

Historically the concept of absolute velocity was commonly believed until the time of Galileo Galilei in the early 17th century. As a naive child, before studying basic physics, it is surprisingly easy to believe in this even these days!

The idea of absolute velocity states that all velocities are fixed with respect to an absolute frame of reference. Galileo showed that velocity is relative to your frame of reference, a principle known as Galilean relativity. This was later fully quantified by Isaac Newton, who also proposed that acceleration is invariant with respect to inertial frames.

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A mistake that I often come across and that is so easy to make: people somehow have a visceral belief that heavy objects fall faster than light ones. Setting aside of course problems of air resistance, this is obviously false, but it seems to be so counterintuitive and I think it is somehow tied to our intuitive understanding of mass as inertia. Since higher mass means higher inertia. People understand this intuitively as it takes more force to push a fat guy than a thin one. But they don't see that gravity is a force proportional to mass, so that more inertia is paralleled by more gravity as well. With as a result the same gravitational acceleration for light and heavy bodies.

I've even noticed the mistake being made by professional physicists in colloquial conversations.

EDIT: I just found this article today, shedding a new light on why misconceptions in physics or science in general are so common and so hard to get rid of.

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+1 for "setting aside air resistance"... it always bugs me when people point out how things fall at the same rate, while neglecting to mention that this assumes air resistance is negligible. Leaving out this clarification I think leads to more confusion, as all someone has to do is drop a rock next to a feather to (incorrectly) conclude that Galileo was wrong. –  Tim Goodman Nov 18 '10 at 3:22
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If you are assuming a fixed earth, then you are correct. But if you let the earth move, then the Earth moves more rapidly when you drop a bigger mass than when you drop a smaller one. In that sense, bigger masses drop faster. This has nothing to do with the equivalence principle of course. –  Vagelford Nov 18 '10 at 17:49
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Seems important to mention here Galileo's famous gedankenexperiment. If you follow the line of thought that heavy objects fall faster, and go split a heavy falling object in two, large one and big one, both should be falling slower than the original. The upper part would be pulling lower part up, wanting to go even slower, hence a contradiction. –  Pavel Radzivilovsky Dec 1 '10 at 9:44
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There is also the complication of buoyancy - a dense object would be heavier at the same mass, and thus fall faster even if air was friction-less. –  romkyns Jan 12 '11 at 1:22
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@Pavel You're ignoring tidal effects. Splitting something in two doesn't really change anything, because the two halves, being in close proximity, experience different tidal forces than those between two objects farther apart. –  ErikE Jan 21 '11 at 18:21
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While the other answers are absolutely correct, they are very subtle in the way they appear. However one very large false-belief in electrical science is that current is taken to be "flowing" from the + terminal to the - terminal in DC current.

While it doesn't really matter which way you choose since you are dealing with close to light speed current, electrons are actually moving from - terminal to + terminal(do not get me wrong electrons are NOT moving anywhere near light speed while drifting. They are moving in the order of centimeters per second.). Hence the current actually moves from - to +.

And it would be interesting to note that not a soul in the professional area of electrical science considers the current from - to +, as it would cause inconsistency with his/her colleagues.

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If current always flows as electrons from - to +, then please explain how the Hall coefficient can be positive in many materials, such as p-type semiconductors. –  Keenan Pepper Nov 18 '10 at 0:02
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The best explanation I heard for this was when they came up with the diagrams they had no idea what an electron was. So they just picked a direction. Turns out that they had it backward and what's actually flowing from + to - is the absence of electrons (holes). –  jcollum Nov 27 '10 at 18:25
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One widespread belief (I think due to popular books such as Hawking's) is that GTR can never ever, ever be quantized and you always obtain infinities and blah blah. Well, it can, in many situations and in many theories. What is actually meant is that GTR is not a renormalizable quantum field theory in the naive way. But this specification is never explicitly pointed out so people get a false impression that quantum gravity is something completely out of the realms of current physics. Well, surprise, surprise, it's not. We can quantize many gravitational effects (such as waves), we understand that black holes have entropy, we understand they produce Hawking radiation and eventually disappear, etc. And assuming that string theory is correct we can predict whole deal more about it.

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Quantum mechanics is way too strange so it can't possibly be a correct description of the real world. Right? I think nothing else needs to be said about this.

Or maybe on a second thought, some more concrete beliefs and their solution are in order:

  1. The physical world has to be deterministic (it doesn't).
  2. Every possible question that can occur to you must have a precise answer by measurement (we observe only what we can, not what we want to).
  3. The collapse of the wavefunction is in contradiction with finite speed of light (no information is being transmitted).
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@Noldorin: Bohr? Really? You wouldn't find a more stringent advocate of quantum mechanics than Bohr (who I think can rightly be called its father). Einstein on the other is a different story. He wasn't able to let go of his belief that physics must be complete and answer everything we want to know. But still, this led to nice results like EPR paradox. So his inquisitive mind arrived at interesting physics even though his prejudices didn't let him accept it :-) –  Marek Nov 17 '10 at 23:33
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@Marek: That's why I put the question mark, silly. :P Who can blame Einstein in any case? I certainly don't. To call him prejudiced is not only arrogant but hugely ironic! I do not wish to continue with this debate, thank you. –  Noldorin Nov 17 '10 at 23:39
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@Noldorin: why would it be arrogant? Just consider that Einstein himself called the inclusion of cosmological constant his biggest mistake, so he himself admitted that he was too prejudiced about stationary universe and unwilling to admit its expansion (but he eventually let go because of Hubble's experimental evidence). Even physicists (especially older ones) can be prejudiced. I am not saying this with any contempt, Einstein was one of the best minds of the human kind. Just that everyone has some prejudice or other. Although in this case it's quite ironic because he helped to create QM :-) –  Marek Nov 17 '10 at 23:47
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@Marek: I think it is not only about lack of experience, but also about teaching QM in wrong way (how often do you hear "No-one knows if electric field really exists or only is our tool to describe how electron moves"? or "If someone says s(he) understand classical probability, s(he) must be lying!") –  Piotr Migdal Nov 18 '10 at 0:23
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@Marek: Yes, it is ironic, as he was one of the great figures in very early quantum mechanics. Still, I don't think that even this day we can say he's wrong! There's nothing to stop some other more fundamental theory superseding quantum mechanics and proving Einstein right. In any case, fair enough, I just suggest you keep a slightly more open mind. :) (I know too many close-minded physicists.) –  Noldorin Nov 19 '10 at 21:39
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A common misconception is that in a double slit experiment, the electrons or the photons go through both slits at the same time and interfere with themselves.

Nothing of the sort ever happens and that idea doesn’t come from quantum mechanics. It is just meaningless to ask the question (from which slit did the particle pass?) in the context of the particular experiment and even more meaningless to say that it went through both. The particle always passes from the one or the other slit if you setup an experiment that asks the question and in the case that you shoot the particles one at a time you always get one hit on the screen and not an interference of portions of the particle.

I think that the misconception has its roots in the wave analogy of the wave function description of quantum mechanics, where you must have a wave passing from both slits in order to have interference on the other side. Of course that picture doesn’t translate to having a particle passing from both slits, but it only states that there exists interference between the wave functions of the two independent events that translates to a distribution of probability for finding a particle on a particular point on the screen. That is a statement of Born’s interpretation that has been recently tested with a triple slit experiment.

One could ask at this point, “Ok, I get it for the electrons, but light behaves like a wave in everyday experience. What happens in that case?”, and the answer is that light behaves like a wave only when you have a large flux of photons and you can go to the continuous field approximation. It is then that the wave-like properties become apparent both for photons and electrons.

Update: A very interesting video on Quantum Mechanics

Update2: QM in your face

Update3: The Feynman Lectures on Physics vol3: Scattering from a crystal (neutrons).

... Let’s review the physics of this experiment. If we could, in principle, distinguish the alternative final states (even though you do not bother to do so), the total, final probability is obtained by calculating the probability for each state (not the amplitude) and then adding them together. If you cannot distinguish the final states even in principle, then the probability amplitudes must be summed before taking the absolute square to find the actual probability. The thing you should notice particularly is that if you were to try to represent the neutron by a wave alone, you would get the same kind of distribution for the scattering of a down spinning neutron as for an up spinning neutron. You would have to say that the “wave” would come from all the different atoms and interfere just as for the up spinning one with the same amplitude. But we know that this is not the way it works. So as we stated earlier, we must be careful not to attribute too much reality to the waves in space. They are useful for certain problems but not for all.

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Actually, what you said is not true. First, electron go through both slits at once (if you want to say something contrary, it's much more about taste or interpretation, that 'fighting with a false belief'). Second, while properties of a single photon are interesting, I would not say it does not behave like a wave (it certainly have many wave-like properties). –  Piotr Migdal Nov 20 '10 at 9:56
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Very strange answer. Actually, it only depends on interpretation whether you think that electron passes through both slits. So this means that it is not a misconception at all. Actually, the most attractive interpretation to me (Feynman's path integral) tells you that the electron travels through all paths and all of them are equally important in determining the final amplitude. In particular, the paths through left and right slits interfere. –  Marek Dec 1 '10 at 0:04
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It seems to me like you are trying to propose ancient preconceptions (like the ones Einstein had) that quantum effects appear just as statistics after letting many electrons through the slits. Well, this is obviously not the case. Every single electron behaves quantum mechanically and travels through all the paths (in path integral view) or is a wave that passes through both slits (in Schroedingerian view). –  Marek Dec 1 '10 at 0:08
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Sorry to step in abruptly, but QM does not tell us ANYTHING about the path of the electron. The ONLY objective, measurable and sustainable thing that QM tells us is the OUTCOME of the MEASUREMENTS. I.e. that electrons will hit the screen in some pattern. All the rest is (non-physical) interpretation and subjective. –  Sklivvz Dec 3 '10 at 21:16
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@Sklivvz: but you are implicitly using one such interpretation yourself. Namely, one that tells you that it's unphysical to ask anything except measurements. You have arbitrarily chosen to make measurement the basis of what is physical (and seems you are not even aware of it). The point is, I can take another interpretation, one that tells me where the electrons went and it also perfectly reproduces all the measurements, so there is no problem with it. It's as valid (and physical) an interpretation as yours is ;-) –  Marek Dec 4 '10 at 16:12
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As a tutor, I frequently have conversations like this:

"So we worked out that if when I toss my pen up at 2m/s, it will go 20cm high. How high will it go if I toss it up at 4m/s?"

"40 cm."

"Well, okay, let's check by working through that equation again..."

[We find out that the answer is 80 cm]

"So when I throw it up twice as fast, it goes four times as high, because it takes twice as long to get to the top, but is also going twice as fast."

"Okay."

"Now what if I throw it up three times as fast? How many times as high will it go?"

"Six."

This isn't a misconception about kinetic energy, so much as a lack of comprehension about what scaling is. When students are missing this concept, almost all of physics is more difficult to discuss.

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@Mark No, not really. They "know" the term is squared. They can instantly recite the formula KE = 1/2mv^2. But they don't really get what that means. They think that the only way to answer questions about what would happen if we throw the thing twice as fast is to plug in some numbers to the formula. The idea of looking at the exponents in formulas and gaining physical insight from that is alien to them. It's natural to you because you know basic physics and math quite well, but to students it is a weird and unusual trick. It's easy to forget how little you knew a long time ago. –  Mark Eichenlaub Nov 19 '10 at 22:25
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Yes - exactly. Building intuition about math is a slow process, but it's interesting to watch a student progress throughout the course of a year. –  Mark Eichenlaub Nov 19 '10 at 22:46
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I would say that for most people, the quadratic scaling of kinetic energy with speed is somewhat of a mystery.

People don't understand how if you go twice as fast, a car accident actually four times as energetic, hence the high number of reckless drivers and deadly accidents.

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A couple of space and sci-fi derived misconceptions:

  • An orbiting satellite needs propulsion and that orbiting is different from free fall.
  • You can actually see a laser beam in free space! I've seen this in my experimental physics class a few years back, before keychain lasers were common.
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And the misconception that orbiting satellites and other space vessels make cool whooshing sounds as they go by at 1/1000th of their actual speed by the pretend-stationary camera. –  romkyns Jan 12 '11 at 1:22
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Seeing a laser beam in free space? Aren't all photons supposed to go straight in free space instead of some of them changing path to land in my eyes? –  Kim Kim Mar 17 '11 at 16:21
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@KimKim I didn't express myself properly: I've seen people believing that. –  Sklivvz Sep 23 '11 at 12:40
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Amazingly, Wikipedia has an article titled "List of common misconceptions". There is a (short) section dedicated to Physics, which mentions:

  • The role of the Coriolis effect in bathtubs and sink drains
  • The role of angular momentum in bicycle stability
  • The "equal time" fallacy in explaining the lift developed by an airfoil
  • Glass isn't actually a high viscosity fluid
  • Composition of air
  • "Lightning never strikes twice"

The Astronomy section has some good ones too:

  • When a star collapses into a black hole, its gravitational pull does not actually increase.
  • Meteorites are not actually hot when they land; usually they are cold. (I would add: the heating of meteors is more due to the compression of the air in front of them than to 'friction with the air' as commonly believed.)

Some that I would add:

  • "Once something is in orbit it is free from Earth's gravity." Even educated people get tripped up on this one; the internet is rife with people suggesting we just "nudge" the International Space Station into lunar orbit. At a much more basic level of misunderstanding, there is the idea that astronauts are "weightless" because they are far away from the earth.

  • "There is a high tide on the opposite side of the earth from the moon/sun because the earth 'shields' the ocean from the gravitational pull."

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Especially the equal time fallacy. Nobody believes you when you tell them it's not true. –  dan_waterworth Dec 15 '10 at 7:24
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Just to clarify the 6 physics examples are things that aren't true (which people believe) - the 2 astronomy ones are things that are true (but people don't believe) –  Martin Beckett Mar 19 '11 at 5:51
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"Long hair grows slower" is due to biological effects

In fact, this is a purely mathematical phenomenon. The bigger average length, the more decrease is caused by each hair fall. This leads to a differential equation $$\frac{dL}{dt} = K - \alpha L$$ that has a solutions which decay exponentially to an equilibrium at $$K/\alpha$$.

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@Pavel: sigh... are you intentionally misinterpreting my statements? I never said single hair is important. Just that your average doesn't account of single hairs (in particular, the long ones) and that the average is not important for this question. In reality what's important is that there is enough long hairs. The girl couldn't possibly distinguish whether she had 10000 30cm hairs or 5000 20cm hairs and 5000 40cm hairs. Your average would come out the same but obviously in the second case her hair would seem 10cm songer. Your model doesn't account for this at all and therefore it's useless. –  Marek Dec 3 '10 at 9:18
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@Pavel: sure, that's what I am saying. But there are many such characteristics and you used just one: the average. And this is the most crudest one and most irrelevant too. –  Marek Dec 5 '10 at 12:38
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"Burning coal for heating is more efficient than electrical due to thermodynamical losses at coal power plant"

This is false, even though it is true that converting electrical power to heat would be even worse. The correct way to heat with a given amount of heat source is this:

  • burn it at high temperature
  • make work with heat machine between Thigh and the environment
  • use the work to power an air conditioner to bring heat from Tenvironment into Troom.

This gives efficiency more than 1 (more heat brought into room than heat produced by burning), and net cooling of the surrounding environment.

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I think one common false belief is that a light mill rotates because photons deposit more momentum on the shiny side (where they are reflected) than on the black side (where they are absorbed).

I find it quite astonishing to see that many people think so despite the fact that a light mill spins to the opposide direction than predicted by that explanation.

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It depends on the vacuum you have. When there isn't a high enough vacuum, the black side is heated more than the shiny side and because of convection of air and some edge effect there is motion from the black side to the shiny side. When there is a high enough vacuum, then the radiation pressure is what moves the mill. So it depends on the light mill. –  Vagelford Nov 30 '10 at 23:04
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Someone should invent a light mill where the pressure inside can be changed enough to observe it working either way. It would be fun to show that to students and non-scientists and ask them to explain. –  DarenW Mar 5 '11 at 21:52
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Something cannot come from nothing

Yes, it can: In quantum field theories, the vacuum is not empty.

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Some sport instructors would tell you:

Running on a treadmill is easier because you only have to jump, while on the street you also push forward

I suggested then that running in a train should be the easiest, by this line of thought. However, it is true that starting to run (or, accelerating) is indeed easier. Or the air resistance, unless there's a slowest forward wind. Or, the randomly changing slope. Also, lack of the air conditioner. That pretty much summarizes the difference.

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Some I have heard:

  • Heavier objects fall faster (this is just plain wrong.) However, bigger and smaller objects in a typical Earth environment would fall at different rates due to air resistance, but actual mass has no influence.

  • Two cars colliding at 60 mph is the same as one car colliding into a wall at 120 mph. I think MythBusters did something on this.

  • Electrons travel really fast around a circuit: actually, they travel really slow but it is similar to Newton's cradle in that a small movement in one ball can transfer the energy to the last one almost instaneously.

  • The laws of thermodynamics have been broken by some guy in a garage with some magnets. Well, no, they still seem to be intact, and a lot of modern science depends on them!

  • That an oscilloscope trace travels faster than the speed of light on expensive high speed analog scopes (~1-2GHz.) This is not quite true: although the beam may sweep the surface of the CRT faster than c (due to the relatively small movement at the neck of the CRT), the trace cannot communicate information faster than c.

  • More related to chemistry, but the fact that water can have "memory" and all that homeopathic nonsense that con arti^H^H^H^H^H^H^H^H homeopaths spurt out.

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It's important to make the distinction about the speed of electrons, but electrons traveling slowly is only half the story. Electrons have a slow drift velocity, but they have a high thermal velocity in most situations. –  Mark Eichenlaub Dec 3 '10 at 9:19
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@Mark Eichenlaub: you can say that they move fast, but they don't travel fast. It's probably a language-related thing, but traveling implies overall displacement. –  Sklivvz Dec 3 '10 at 21:11
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Wait, what's wrong with the colliding cars? Shouldn't it be the same in all frames of reference? It certainly is the same in the case of an elastic collision. –  Greg Graviton Dec 4 '10 at 17:49
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Ah, because the energy is used to deform both cars. So, that would be like a car with 120 mph against another car instead of a wall. Would it? Kinetic energy is quadratic in velocity, but something weird happens when you switch the frame of reference. –  Greg Graviton Dec 5 '10 at 13:09
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@Greg: here's an analysis of the crash that might interest you. Also see this question. –  David Z Dec 15 '10 at 0:06
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If you somehow manage to BREAK a law of physics, the universe will vanish!

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Fortunately if you do it will be replaced by a backup copy –  Martin Beckett Mar 19 '11 at 5:59
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"Summer is when the Earth is closest to the sun, and winter is when it's furthest away."

It's true that the Earth's orbit is slightly elliptical, but the effect of this, as far as seasons, is very small. For one thing, this wouldn't explain why the sun rises and sets at different times in different seasons, and if this were true, the whole planet would have summer at the same time.

The seasons are actually caused by the tilt of the Earth relative to its orbit around the Sun.

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Why do I have to learn this law when they change it every few years?

This has to do with a fact that some (actually, a lot of) people believe that the progress in physics is done in form of revolution and (in particular) that one day we might find laws that will contradict everything we knew up till then.

Well, if one looks closely on history of physics, it should become apparent that progress was always just evolutionary. Even when some idea needed a revolution in the way people think (as with SR and QM) it always turned out to be just a generalization of our previous ideas (so both SR and QM have nice classical limits which coincide with Newtonian mechanics).

Barring the useless philosophical views (like we might live in Matrix or we don't know whether the sun will rise tomorrow for sure) it's pretty certain that our universe is a comprehensible place and our theories are just better and better approximations to the reality. So it will always be useful to learn Newtonian mechanics, even one million years from now.

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You need something more to get something more

This is about emergent macroscopic properties of microscopical laws. Some people can't understand that statistics is powerful enough to make seemingly random heap of molecules suddenly show macroscopic properties like being solid or being magnetic and they think some hand of god is required to make this happen.

The best illustration of a contradiction to this principle are living organisms. They consist of nothing else than few physical laws all the way down and large number of molecules. All that was needed was statistics and natural selection.


I will agree though that it is quite amazing all of our nature can spontaneously emerge just from particles given enough space and time.

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@Ron: I am not saying it's obvious, only that it is possible (i.e. you don't need God to create life). And it is certainly the only scientific explanation we have, so what needs to be done is "only" better quantification of relevant processes. –  Marek Aug 16 '11 at 17:15
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Classical mechanics is boring and mostly solved

...especially in case of fluid dynamics (-;

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I never met a person who would think this. If you don't know anything about mechanics then you obviously can't think it is boring. And if you do know it, just double pendulum or three-body problem should convince you that it's far from simple. I wouldn't call fluid dynamics classical mechanics though. While it is classical, it is certainly not mechanics but rather a field theory. And that is the main reason it is hard. –  Marek Dec 3 '10 at 12:46
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I would say most physics undergrads think this is so. This is closely related to "anything that can be solved analytically, already has" –  Pete Dec 3 '10 at 16:01
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The most common misconceptions are about gravity:

(1) Gravity turns off at space-shuttle orbit distance because the astronauts are weightless

Gravity is at about 80% strength compared to the surface of the Earth. The astronauts are weightless because the shuttle is in free-fall (orbit). If there was no gravity, the shuttle could not orbit.

(2) Gravity is generated by the spinning Earth. If the Earth stopped spinning, gravity would turn off.

Gravity is generated by virtue of the Earth's mass and the mass of the object; the two exert a mutual pull. There are some smaller effects associated with the spinning Earth (e.g. the Coriolis effect), but gravity would still work fine if the Earth stopped spinning.

There is another great misconception about the force of impact between a truck and a small car in a collision:

(3) The truck exerts a greater force of impact on the car, than the car on the truck

While the damage can be certainly unequal, the forces are.

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(3) is not really a misconception if force is understood in a colloquial way (normal people don't really use words like force and work in the physical sense). In particular, truck will have a lot more momentum so the situation is certainly asymmetric. But otherwise I like these. +1 –  Marek Dec 3 '10 at 16:25
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the belief that cartoon-universe rules apply to falling objects:

as embodied by the statement that, "if you are trapped in a falling elevator, you can avoid destruction by jumping at the last moment, so that when the elevator hits, you are in the air, and only fall the last inch or two."

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It really bugs me. I've even heard from some quantum mechanics lecturers that they think quantum entanglement implies faster than light communication!

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Quantum theory is not non-local (and I am not really sure what you mean by that intergalactic invasions, @Pavel)! This could be put as another misconception stemming from EPR paradox. What EPR (or rather Bell's inequalities) say is that quantum theory is either non-local or incomplete (in a sense of absence of hidden parameters). We have good reasons to think it is local (e.g. QFT has to obey locality if it is to make any sense) so what these experiments actually concluded is that there are no hidden variables (i.e. QM is not just statistics). –  Marek Dec 5 '10 at 12:34
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@Pavel: okay, done. It's interesting but it's still just the good old correlation of entangled spins, nothing more. Calling it non-local (or second-best, or whatever) is just confusing and precisely the reason why people think there is superluminal communication going on. –  Marek Dec 5 '10 at 17:55
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I hear from time to time people highly educated and skilled in Physics (unlike those believing that heavy objects fall faster than light ones) making the following claim:

... quantum mechanics indicates that certain physical quantities can take only a countable set of discrete values. Consequently, many current approaches to foundational questions in physics and cosmology advocate novel discrete or 'digital' pictures of nature.

("Is Reality Digital or Analog" essay contest at FQXi)

The discrete spectra of some quantum observables do not imply/suggest that nature, in particular spacetime, is fundamentally discrete. The spectrum of a continuous operator acting on Hilbert spaces [which is a topological (vector) space, hence is continuous], often has a discrete part. This has nothing to do with spacetime being discrete. If it will (eventually) turn out to be discrete, it will be for other reasons.

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If you are riding a bicycle and you turn the front wheel to the left then the bicycle will steer to the left.

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This is tongue-in-cheek!

That an emptying tank with a nozzle pointing downwards would actually experience a force!

(see http://arxiv.org/pdf/physics/0312087v3)

:-)

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That if an object is moving, there must be a force propelling it in that direction. Students very commonly think that forces cause objects to have velocity, rather than the fact that forces cause objects to change velocity.

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Related to this, it really seems to confuse a lot of intro-level physics students when acceleration and velocity vectors are pointed in opposite directions. One of my standard quizzes when I taught freshman physics was to toss a ball straight up and down in the air, and then hand out a position vs. time plot and ask them to sketch the acceleration and velocity vectors at several key points. (neglecting air resistance). Always gets an interesting mix of answers. –  Tim Goodman Feb 21 '11 at 17:24
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Here's a false belief. Everything your physics teacher says is true. Because physics teachers would never mislead you over how many states of matter there are or how lift is generated on a wing.

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That the PI Team is the best group to design and implement their own data systems, rather than bring in people experienced with IT security, data modeling for reuse by other groups, and other data informatics issues.

Related misconception: that their data is so different from other data that a data system must be designed from the ground up for each new experiment.

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