I thought of that question differently. Ignoring all the engine effects and only considering the rotation of the tyre on the ground. If you follow a path on the tyre it meets the ground and pushes on it. If there was no friction the tyres would spin on the spot. Once you bring friction in the tyre's have something to 'push off' from. This is probably the wrong way to look at it though Please Register or Log in to view the hidden image!
<b>About this tire friction business:</b> If the tire and road are sufficiently coarse, then the interaction could be a simple surface normal to surface normal interaction. The normal force between the tiny irregularities could provid a push without friction. Although, this is probably also lumped into the vague notion of friction. I got #6 and #8 wrong because I didn't know that 0 K is impossible. I'm terrible at statistical mechanics. Can someone explain why 0 K is impossible, in principle. I got #13 and #14 correct, but I disagree on the reasoning. I don't think that F<sub>grav</sub> has anything to do with the appearence of weight (as it does not truely exist). I think that the appearence of weight comes from the non-trivial acceleration, regardless of the cause. I got #23 correct, but only because I assumed that the road was flat (not declined), and that the road and tires were sufficiently smooth so that the only transverse force between their aggregate would be considered friction. I got #25 correct, but only because I assumed a Galilean context. I got #28 correct, but only by luck. I didn't know whether there was some property that determined the ability of an element to obtain some phase, or, like the explanation said, any element can obtain any phase, given the sufficient condition for doing so. I thought about dry ice, that it sublimates, but then again, dry ice is not elemental, and there may be other pressures that yield a liquid phase for CO<sub>2</sub>. I got #32 correct, but only in anticipation of the interpretation of empty space. I thought the jury was still out on the issue of whether empty space even has meaning when speaking of things on the subatomic level. I got #33 correct, but I disagree with the reasoning. The car is not a completely closed conductor, nor is it a perfect conductor. My reasoning was based on the relative conductivities of the metal frame and the interior (including the people). These represent two paths to ground. Statistically, the current in the lightning bolt has a small probability to flow through the interior (and thus, through the people), but averaged over the charge flow of the whole bolt, it would not represent very much current. In other words, to simplify the language, the metal frame is like a 1 Ω resistor, the interior is like a 10 MΩ resistor. The combination is like a current divider. So, assuming these numbers are somewhat representative, only about 0.00001 % of the current would flow through the people, and that distributed throughout their bodies means even less would flow through their hearts. I got #34 wrong, but only in anticipation of a misuse of the word "voltage." But, of course, like in the energy basics section, force and energy are not the same thing, and neither are force and potential. I got #38 wrong because I don't have my glasses on, and therefore made the misinterpretation that "mL" said "mm."
Hello Anders Not no friction. The way the question was structured was to give the impression that the road is the cause of the friction. The cause of friction is the weight of the car on the road. The propulsion mechanic of the car over comes this friction and the car moves. The greater the mass of the car, the greater the friction to be over come. The way the question was structured was to imply that the road produces more push if the car has more mass. The push was in inverted commas but roads can’t apply a horizontal vector. The equal and opposite and reaction is between the pistons and cylinder head, not the tires and the road on the plane of the surface of the road. Regards leeaus
I agree.... Some of the questions were worded strangely or weren't informative enough :-/ Oh well, Im still retarded.
I got 80%, 8 wrong. I am so ashamed. Please Register or Log in to view the hidden image! I aced all the important tests in my classes though (as in 100% cept 1).
friction moves car Agree with AndersHermansson that friction is one of the most important factor that makes a car moves (accelerate or decelerate). Car on the slippery surface, say no friction at all, could not accelerate or decelerate even with the most powerful engine or braking system. Regards, paul
I'm confused about #6. If I release X joules of energy when I burn my gasoline and I have the ability to prevent unwanted heat transfer, then why wouldn't I be able to use all X joules to move the car? Where does the energy go if it's not going into moving the car, friction, or heating things that I don't want heated?
Your Total Score is 55 % I never took a physics class in my life. Actually, the only thing that comes close to one would be a book I've done on static physics. I did that about 2 years ago and trust me when I say it was mostly basic things. It was a third of the high school introductory physics class so I'm not too worried about my result. In fact, I'm quite satisfied with it... Please Register or Log in to view the hidden image!
82.5% Missed #6 because I haven't taken thermodynamics yet (although it starts this week Please Register or Log in to view the hidden image! ) I'd also argue #8, thermal energy can do work just as well as mechanical, right? Missed 13 because I wasn't thinking. 29 because that's clouds are generally explained as water vapor and I don't care enough about them to find out on my own whether it's true. 30 because I didn't assume "solar system" meant "elliptical orbits" 34 because of the reason they gave in the explanation, shoddy labelling in textbooks. And 35 because car batteries scare me. Not too awful I guess, considering the lack of sleep last night.
I'm with you an this one. The explanation said something about the <i>impossibility</i> of 0 K, but that seems to contradict the <i>possibility</i> of no unwanted heat loss. I interpretted "if no unwanted heat loss" as "if you have a 0 K cold resevoir." Of course, if you had a 0 K cold resevoir, then the engine probably wouldn't be able to move, and in that case the efficiency would be 0.
Wouldn't 'no unwanted heat transfer' mean that I could transfer zero energy to my cold reservoir, thus making it 100% efficient? I thought that heart engines are always somewhat inefficient because you always transfer some energy to the cold reservoir rather than using it to do work…
No its not about conservation of energy but about the second law of thermodynamics and 'entropy'. It basically says that heat cannot be used (100% efficiently) to do work (i.e. be tranformed into another type of energy.)
Right, but it's saying that even if you could prevent unwanted heat transfer that your engine still couldn't be 100% efficient. Again, where is the energy going if it isn't being lost to friction, motion, or unwanted heat transfer?
72.5%Please Register or Log in to view the hidden image! Guess I'm not too dissatisfied with that. May have got one or two more but I found some of the questions a little confusing. Yup I fell into the 'friction trap' and I was blindsided by the fact that I can drive around town all day and as long as I come back home my average velocity will be zeroPlease Register or Log in to view the hidden image! Better stick to medicine I guessPlease Register or Log in to view the hidden image! Dee Cee
That's what I thought the Carnot efficiency evaluted to for a 0 K cold resevoir as well. But I read the explanation, and it said that the reason you can't have a 100% efficient engine is that 0 K cold resevoirs are impossible. I sure would love for someone to explain that to me.
well sure, that is the zeroth law of thermodynamics, you cannot reach 0 K. but if you could, it would not mean 0 efficiency, it would mean 100% efficiency.
<i>that is the zeroth law of thermodynamics, you cannot reach 0 K.</i> Isn't that the third law? The zeroth law says if A is at the same temperature as B, and B is at the same temperature as C, then A is at the same temperature as C.
I thought that the zeroth law said that heat flows from high to low temperature. I didn't think it said anything about the impossibility of 0 K in the first place (i.e. before we even talk about heat). At any rate, I think you misunderstood me. I was speculating that, if you had an engine (probably made of metal), then, at 0 K, would it not freeze up, or something.
