Cheesesteak
Yeah that volatage question was one of the two I got wrong.
If I remember my physics correctly, another term for voltage is an electromotive force.
100% (which I damn well better get being as I am finishing my doctorate in aerospace engineering).
I also got the cloud question right for the wrong reasons, but I still think I am right. OK, they were going for the difference between water vapor and droplets, fine, but even if they had said water droplets or crystals it still would not be true.
Observation: Clouds float in the air
Conclusions: The density of a cloud is approximately the same as the surrounding air.
Reasoning: If the overall mass of a cloud is the sum of the mass of the air Ma and the water Mw, then the density of the cloud is (Ma+Mw)/V. This should be appoximately the same as a nearby volume of air which consists of the same amount of air plus the bit that is replaced with water in the cloud (Ma+Ma2)/V.
So,
(Ma+Mw)/V ~= (Ma+Ma2)/V
Since Mw >> Ma2, (due to relative densities) the above equation can only be true if:
Mw << Ma
Thus most of the mass of a cloud is in the air.
Or rather, EMF is measured in volts
If they worded it so that the hammer and feather fell in a perfect vacuum, the answer would have been the same.
Except that in a perfect vacuum there would have been no “virtually” in the question. That word “virtually” was misleading - harrrrumph.
100% baby! 
I guess that physics minor was of some use. That said, I did sweat the wording on a couple of questions, specifically #8 and everybody’s favorite, #29.
The apropos explaination for #8 is that the question speaks of ability to do work; work is, of course, the displacement of an object with respect to a given energy level. Without some kind of heat engine, 1000J of “thermal energy” (i.e. molecular kinetic energy) you aren’t going to do anything but heat up your target object. In order to achieve directed motion, you have to turn that heat into some form of mechanical energy; a Stirling cycle engine, a ramjet, a thermal sail, steam rocket, something. And as others have pointed out, all heat cycles have inherent thermodynamic limitations on efficiency; it simply isn’t possible to extract every last erg of “heat” from a reservoir. At the very least, you are going to be limited by the 3.7 Kelvin cosmic background blackbody as your ultimate low temperature (rejection) reservoir.
#29 represents a disconnect between technical language and common use; we often talk about water and other liquids being “vaporized” far below their gas temperature/pressure curve, but what we mean is a mist–finely divided droplets that can, under the right conditions, remain suspended in air indefinitely. Technically, however, vapor refers to a substance in the gasous state near or at its phase transition to liquid or solid. Under some conditions, water (and substances) can remain liquid even though they are above the phase transition temperature (such as superheated steam escaping from a radiator) but it will either vaporize or condense rapidly to achieve equilibrium.
There were a few other questions that were, at least to me, clear in terms of the answer they were looking for but were worded in a way that was indistinct or not rigorous, such as #13. The astronaut in freefall is weightless, in the sense that there is no upward force applied to her and no force she can measure. The reason isn’t a lack of gravity of course–gravity is what keeps her in orbit rather than flying off–but the fact that the gravitational attraction between the abandoned spacewoman and the Earth is balanced by her momentum directed tangential to her orbit. That’s the general problem with stating physics questions in English (or any natural language); the language is indistinct and multivalued. Stating these questions as diagrams, equations, or Peano algebra (as is done in any technical treatment) eliminates the normal ambiguities of natural language…and lets you get into the deeper ambiguities of the natural world.
Stranger
And water can remain liquid below the normal freezing point if cooled slowly and without being mechanically disturbed. An airplane flying into a supercooled cloud suddenly is covered with rime ice and that can be disconcerting, to say the least.
I got 55%. Now I feel stupid.
I read a lot of theoretical physics (Greene’s Elegant Universe, etc.).
Dammit.
82.5 not good for me. Sigh.
One question I have. I believe Albert Einstein realized that an object in free fall felt zero force. That is the whole point. As the object crosses into different time shells the energy has to change (conservation of energy and all that) and so the velocity goes up. But the idea that a satellite experiences a force is Newtonian physics. Of course it experiences an acceleration since the direction changes, but it stays in the same time shell around the earth and so experiences no change in energy. Perhaps I don’t understand Einstein’s theory… Can anyone explain the explanation?
You & me both! I also got 72.%, having never taken a physics course before. Oneof these days I’d like to; but it’s not my top priority.
Campion, may I suggest that after you take a physics course, you take a probability course?
Daniel
Dangit, teach me not to notice the second page. Sorry, campion.
The magical engine one bugs me, too: I dispute that a magical engine operates according to the rules of physics. Maybe this comes of DMing for people who try to explain the physics of building a Faraday cage to resist lightning bolt spells.
Daniel
85%.
Are they using the word “force” the same way in their explanations of questions 23 and 34?
I’m not sure what you mean by “time shells”, but with respect to general relativity, Einstein replaced the ambiguous “gravitational force” of classical (Newtonian) physics with a space-time continuum described by a Lorentzian manifold which becomes “curved” in the presence of mass or “stress-energy”. In other words, matter causes space to curve, and like the classic example of the bedsheet and the bowling ball, the curvature causes other mass and energy to take a curved (geodesic) path. This, in addition to the realization (from special relativity) that there are no objective inertial reference frames (nonlocal observers see events happen on different timeframes relative to their position and velocity to the event), the inclusion of the relations for electromagnetism bearing Maxwell’s name, and the confirmation that light moves at the same speed in all frames comprise the essential conclusions of general relativity, though they certainly don’t begin to exhaust the possibilities rising therefrom.
In the sense that curved space replaces “gravitational force”, then no, objects described by general relativity don’t see forces due to gravity; however, this “acts” just like a “real” force, like the inertial force of rocket thrust, and thus, we abstract it as a force for the purposes of mechanics for all purposes outside of relativity theory and particle physics. Since all “forces” (gravity, electromagnetism, nuclear strong and weak) are due to underlying interactions in some cosmological manifold beneath our perception (according to superstring theoriests and the like) or are the result of fundamental particle exchanges to small for us to see (particle physicists), or some hybridization thereof, all forces, even [thread=299054]mechanical contact forces[/thread], are abstractions; there are no little blue arrows with attached tags saying “100 lbf” pushing blocks up slopes.
To get back to your first statement, an object in free-fall sees no net force; that is, all forces are in balance. In the case of an object in free-fall orbit, the gravitational force is opposed by the inertia of the object. The resulting acceleration acts to continuously curve the path of the satellite about the planet in a conic orbit; hyperbolas and parabolas for those in fractional escape orbits, and ellipses and circles for captured orbits. And the velocity of a satellite is non-constant except in the degernate (and naturally unlikely) case of a circular orbit. Objects in elliptical orbits speed up as they approach the planet and slow down as they fly away, as described by Kepler’s Second Law of Planetary Motion.
Stranger
Well, not exactly. #34 has at least two conceptual fallacies in it (making the correct answer “false”). The first is that voltage is a force; it’s not, and the units (volts) clearly indicate that. Voltage represents a potential for action; if you were to make an analogy to a waterfall it would be represented by the height of the fall, and current/amperage would be the volume of water. The second fallacy is that electricity is propagated by electrons continually flowing through the wire like some kind of plasma. Conduction in wires is quite a bit more complex than can be explained in a couple of sentances, but in essence the property of conduction in metallic lattices it due to the nonlocalized sharing of electrons (sort of like a covalent bond, only it’s more of an orgy than a relationship) and when the conductor becomes charged the electrons all hop up into excited states, like Notre Dame fans celebrating Jesus’ reception abilities. (Appearently the Savior is quite the quarterback.)
This can be kind of confusing because voltage was traditionally referred to as electromotive force (emf), even though it’s not a force and the units are written in volts. Nonetheless, this convention continues through the present day and is presented in undergraduate circuits and EM physics textbooks, often without clarification.
Rereading the test I’d say that anyone scoring an 80% or better probably has a reasonably good practical grasp on the physics and is more likely tripped up by the wording or conceptual ambiguities like the above. Not good enough to score a 800+ on the Physics GRE, but good enough to nitpick Armegeddon, Die Another Day, and Intuitor’s #1 Pick as worse movie physics ever, The Core.
Stranger
It threw me, too, at first, and I’m perversely fascinated by the fact that the answers are skewed to “false.” Still, I gots me my 50% and I’m wearing it with pride. Even though it’s pretty much the lowest score posted. Everybody has their strengths and weaknesses, and the world would be a much duller place if we all could do the same thing.
And, besides, what is physics possibly good for? It’s not like anyone actually uses it in real life.
85% - better than I expected. Not bad for a liberal arts major. 
flight, Stranger, congratulations!
C’mon in; pull up a chair.
A very surprising 90%, not bad given I’ve done no physics since high school (over 30 years ago). The ones I got wrong were:
8 I can live with, of course you need to lose some energy when you convert from heat to mechanical and I’m not sure what my thinking was there.
14 depends on your parsing of “directly responsible”; I thought the force on her feet was only indirectly responsible for her perception of weight, as that was mediated through her nervous system, brain, etc etc … I guess I should have reminded myself this was a physics test not a biology one.
17 I don’t 100% get; I was always taught velocity = speed and direction. The directional portion of her velocity adds up to zero of course, but the speed portion of her velocity does not, it’s always 15mph and so doesn’t cancel itself out.
23 I don’t agree with; friction is not a force. Certainly the car won’t move without it, but it doesn’t follow that it’s a force. Let’s take an example of me standing on roller skates. I slowly swing my arms forward then rapidly fling them back; I move forward. Friction is what stops me moving backwards during the initial swing, but in no sense I can divine is it the force that moves me forward; in fact the less friction there is at that juncture the faster forward I go.
82.5% I didn’t like the wording on a few questions. I mostly had the concepts right but the semantics were what they were arguing in some of the ones I missed. I’ll take my lumps on a couple though. I forgot about velocity requiring a vector, for instance.
87.5%, which is pretty terrible as 25% of my degree is in physics, I got 8,15, 20,31 and 34 wrong. Lots of brain farts.