CuriousBanker

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1. Why is 30 the cut off for z vs t distribution?

Why not 29 or 31 or 35? Is there like a logical reason for 30 or was it just decided upon by somebody?
2. Why is 30 the cut off for z vs t distribution?

I am taught if it is over 30 observations it will be probably normally distributed. How the heck was 30 chosen as the magic number?
3. Why do we use n-1 for sample denominator?

Hi. So I understand the concept of degrees of freedom, as in we are not free to choose x many variables. Like if there are three numbers and the mean is 20, the degree of freedom is 2, because if we choose 2 numbers, the third one is defined. I don't get, however, why sample's of a population use n-1 in the denominator, when populations only use 1. Lets say this is a population: 1,3,5,2,7,8,6,9,2,7. The mean would be 50/10=5. The variance is 7.2. Let's take a sample of this population. 5+7+6+8+7. Let's take a sample of this population. 5+7+6+8+7. The mean would be 33/5=6.6. The variance, using N IN THE DENOMINATOR (which I know is incorrect), would be 1.04. With N-1 as the denominator, the answer is 1.3. I fail to see how 1.3 is any more accurate of the population than 1.04, since neither is anywhere close to 7.2. I guess because it is ever so slightly closer to 7.2? I don't really get it conceptually. Thanks in advance. Also, if the sample was 1,3,7,8,9. The mean would be 5.6. The variance using N would be 9.44. The variance using N-1 would be 11.8. So in this case, N is actually closer to the real deal of 7.2, than n-1 would be
4. How do goosebumps create insulation?

Got it, thanks!
5. How do goosebumps create insulation?

I understand now, thanks. Now I am wondering though....how does the hair 'trap' the warm air?
6. What happens to molecules as an object stretches?

Would an into to chemistry textbook suffice?
7. How do goosebumps create insulation?

Well with looser pants there is actually more material then tighter pants. Wouldn't a better analogy be comparing tight pants that cover all of your skin to loose shorts? When the hair is laying flat on your skin it is "tight" and covering all of your skin But when it is raised, although it is "looser", it is also covering less area
8. How do goosebumps create insulation?

So the hair stands up. This is said to create more insulation That makes no sense Yes, the hair stands up, making the barrier to your skin greater. But before it was standing up, it was laying flat, covering all of your skin. When it stands up, it leaves more of your skin exposed, because it was covering the whole surface area, and now it is covering less of the surface area, but a higher length. Wouldn't these effects just cancel each other out?
9. What happens to molecules as an object stretches?

Kind of. But I am still a little confused. If the macro changes, the micro has to change to. If all of the molecules stayed exactly the same, then the macro could not change...right? How could a bigger picture change without the smaller picture underlying it also change if the bigger picture is made up of the smaller pictures?
10. What happens to molecules as an object stretches?

I don't understand. Aren't the fibers made of molecules like everything else?
11. What happens to molecules as an object stretches?

Let's say you have an object, such as a shirt, and you pull it in all directions. After a while it will become permanently stretched out. This is obvious on an every day level. But what is happening molecularly? Are the molecules becoming pushed further and further apart from each other? If so, how are they still attracted to each other enough to stay bonded?
12. How does moving your legs forward propel you in swimming?

I understand what you are saying about breaststroke but not about flutter kick. Maybe I'll never understand. When you use a handheld fan you push air in one direction then you push air in the other direction. Net, you are moving back and forth but not going anywhere. How does it help you maintain forward movement? Obviously I'm missing something. I don't know what. Thanks for trying
13. How does moving your legs forward propel you in swimming?

Yeah the motion of your legs propels water backwards moving you forward. But what about when you have to bring your leg back forward. Wouldn't that propel you backward? As for the snake analogy I don't really get how snakes move either Also if the legs scissoring pull you up wouldn't they push you down when they go the opposite way?
14. How does moving your legs forward propel you in swimming?

Ok, so I know why your arms move you forward in swimming. You are pulling the water back which pushes you forward a lot because there is a lot of resistance, and then you are moving your arm forward in the air which offers little resistance to push you back. So the amount you push back in the water is greater than the amount the air pushes you back when you move your arm forward. But what about the legs? There are two things I am confused about: 1)How does moving your legs vertically propel you forward at all? I don't know how a downward kick can move you forward. If I were to kick the ground right now, it would not move me forward. 2) If kicking your legs down propels you forward, shouldnt bringing your leg back to the original position move you backward? For instance, if I were to floor the gas pedal in my car while it was in drive for 5 seconds, it would move me a distance forward. If I were to then throw my car in reverse and floor the pedal for 5 seconds, it would move me back to my starting point. So how come kicking down propels you forward, while kicking up does move you backward? Sorry if I did not explain my question clearly enough.
15. Can I teach myself?

Hello again, So, I have decided going back to school does not make financial sense, and I don't think science is the career path I want. I am currently studying for my CFA. However, I am very interested in physics and chemistry. My girlfriend was a chem major in college, and sometimes, I just want to know whatever the heck she is talking about, haha. Plus I find science fascinating. My question, is it possible to learn as a hobby, without a lab or teacher? I know I can use textbooks, the internet, and my girlfriend, will that be enough? I am 23. When I am done with my CFA, is it something I can learn? If I spent, lets say 15 hours a week just learning for fun, would I be able to get the gist of it in 2-3 years? I already have the calculus, I know I can teach myself linear algebra. So I would need the two intro textbooks, organic, inorganic, Pchem, biochem...what else? Later in life I would like to get into science as a career, if and when I already have money. So I plan on teaching myself physics, chemistry, and biology (genetics). Are these possible to learn by researching them myself? I don't need to be an expert, but is it reasonable to think that 5-6 years of studying 15 hours a week as a hobby, I could get a good grasp of the above mentioned chem subjects, and also some physics (waves, thermodynamics, electricity magnetism and fluids, quantum mechanic, nuclear physics)? Or is it a pipe dream and I won't really be able to learn without school? Don't get me wrong I know these subjects are no joke and you cant just pick up a book and expect to know everything, just wondering if I should even bother. Thanks in advance.