# Search results

1. ### Positive Definite Matrix - I think this is correct? Just want reassurance.

Homework Statement For what b is the following matrix positive definite? (\stackrel{1}{b} \stackrel{b}{4}) (Sorry for the strange way I have that matrix represented) Homework Equations For a positive definite matrix x^{T}Ax for all nonzero x in \Re^n The Attempt at a Solution x^{T}Ax =...
2. ### What's a good home built gift for an aerospace engineer?

Hi, I've got a friend who's birthday is coming up. She's an aerospace engineer and I was thinking it would be fun to build a small project for her but I'm not sure what. I want it to be specifically aerospace related. I've seen designs for how to build a pocket jet engine, but I wasn't so...
3. ### How are the percentage contents of the universe calculated?

Specifically I'd like to know about dark energy. How do we know it's 74% of the universe? I'm really looking for a specific explanation/derivation. I've searched a few places and all I've found so far has been people who say "due to Friedmann's equations" or "the WMAP measurements show that"...
4. ### Space Station Vs. Moon Base

Why is the idea of putting a station in orbit more appealing then putting on on the moon? I understand the want to test things in low gravity. But what about cost wise? I know that the ISS has to change it's direction every now and then to avoid random junk floating in space, and I understand...
5. ### Why does zero viscosity allow superfluids to climb?

Since there are no frictional forces acting on the superfluid my intuition would tell me that there is no way for the liquid to "climb" the sides of a container. Obviously though, intuition isn't the best thing to go by when it comes to superfluids. I'm just wondering: what mechanism drives it...
6. ### Need Suggestions For An Experiment - Upper Division Undergraduate

Hello, During the spring semester of school that is just beginning I will be a junior in physics at the University of Minnesota. I'm taking a physics class called Methods of Experimental Physics II in which for the majority of the semester groups of two will be working on an approved...
7. ### Why doesn't the asteroid belt block our view of the jovial planets?

Why doesn't the asteroid belt block our view of the jovial planets? I'm assuming it's because the asteroids are just too few, far apart, and small to affect our view, but I'm not sure and I'd like to know. Thanks!
8. ### How Do You Find A Central Force Orbit?

If you are given a central force field and an initial velocity of a particle in this field, how would you go about finding the orbit of the particle in polar coordinates? Thanks for you help and time.
9. ### Why Dark Matter and Dark Energy?

Hello, I'm pretty uneducated on the topic of dark matter and dark energy, but I know a little. I was just wondering why dark matter and dark energy are the more "expected to be true" theories then many others currently. I'm not suggesting that the following theories are right or wrong, I'm...
10. ### Moment Of Inertia Of Sphere At A Distance

If a sphere is at a certain radius from the axis of rotation greater then the radius of the sphere can you just take the moment of inertia as a point mass, I=mr^2? Thanks for your time.
11. ### White Dwarf Cooling Questions

Perhaps this should be on the homework forum but I'm not sure, so I put it here. How do you calculate the cooling time scale of the surface of a white dwarf and the surface of the core of a white dwarf? I have an equation for the cooling scale of a white dwarf in general but I'm not sure how...
12. ### Mechanics - Show Forces Are Conservative

Homework Statement Show that the following forces are conservative: a) \vec{F}=c(\vec{\omega}x(\vec{\omega}x\vec{r})) b) \vec{F}=d(\vec{\omega}x\vec{v}) where c and d are constants. Homework Equations There are probably more but I think helpful ones include: \vec{\nabla}x\vec{F}=0...
13. ### Why is the acceleration on a point on a wheel what it is.

Hello. My brain doesn't seem to be working at the moment. If a wheel's center is traveling at V0, why is the acceleration of a point on the edge of the wheel equal to V02/r where r is the radius of the wheel? Thanks for your time. Edit: Oops should have have put a question mark in the...
14. ### Specific Energy Generation of the sun.

Homework Statement (a) Compute the specific energy generation rate of the Sun as a whole. (b) Assume a human of weight 100 kg has a “luminosity” of 100 W. Compute the specific energy generation rate of a this human. Homework Equations Need help with the equations part. The...
15. ### Separation Into Differential Equations

I would use the entire template except this question is very simple and does not require all of it. Homework Statement How do I separate \frac{X''(x)}{X(x)}+\frac{Y''(y)}{Y(y)}=\sigma into ordinary differential equations when \sigma is a constant. Thanks for your help!
16. ### Seperation of variables in the 2 dimensional wave equation

[SOLVED] Seperation of variables in the 2 dimensional wave equation I'd like to apologize right away for the terrible formatting. I was trying to make it pretty and easy to read but I guess I'm just not used the system yet and I had one problem after another. As you'll see at one point the...
17. ### Charge In Schroedinger's Equation

This is a simple question which I'm sure has a simple explanation. While mass of the particle is explicitly included in the Schroedinger Equation, the charge is not. Why isn't it? Is it included along with the potential energy or....?
18. ### Magnet Wire Current Limit

Hi, I have 18-awg enamel magnet wire and I was wondering what the maximum current I should use is such wire is. Thank you.
19. ### Rotational And Vibrational States

I apologize for disregarding the template, but it did not seem necessary or practical for my question. In molecules which has more energy, a rotational or vibrational state (first state for both)? I've just calculated the vibrational and rotational energy of an HCl molecule and my results...
20. ### Which Electromagnet Design Works Best?

I have magnet wire and I have steel with extremely high iron content. I was just wondering, if I were using the same amount of wire and metal no matter how I built it and I were just making a straight cylindrical electromagnet, what would be the more powerful design? More towards the short and...
21. ### Why isn't it possible that gravity is repulsive at long distances?

Hello. I'm still only an undergraduate studying physics, so there's still much I don't know. Hence why it's probably likely that I can't figure out the answer to this question myself. Anyway, I've looked into the subject of dark energy a little bit and from what I've seen we still know very...
22. ### Medical Magnetic Fields Versus Health

I know high magnet fields can be dangerous to health. My questions are 1) at what point does a magnetic field become powerful enough to be actually cause problems with human health and 2) why exactly is it hazardous at high levels? Thank you for your time.
23. ### Soft Iron

If I were to go to a random hardware store and look at some iron, how would I be able to tell how soft or hard the iron was? Also, is there any other cheap magnetic material that would work better then iron to make an electromagnet with? Thank you for your time.
24. ### Hello, I am currently planning on constructing an electromagnet.

Hello, I am currently planning on constructing an electromagnet. Its purpose is entirely for entertainment. I was hoping to make two of them that I could attach handles and switches to so they could be used for hanging/climbing underneath some random thing. If possible, it would be useful if...
25. ### Automobile Speaker Voltage

What voltage do car speakers typical run at? Thank you for your time.
26. ### Laser Amplified By Plasma

Recently I read the thread "Giant Solar Laser" in the General Astronomy Forums. The author, Ruslan_Sharipov, writes I was just wondering if someone could explain specifically how a laser will be amplified when passing through plasma. Thank you for your time.
27. ### Electric Car Engine Weight

Hello everyone. Currently my brother and I are considering converting a gas engine car into an electric engine car. I'm still not exactly sure how well the vehicle we have chosen for this project will work. The main issue I'm worried about is weight. So, I was wondering if, in general, a...
28. ### Battery Recharge In An AC Circuit

Hello everyone. This should be a fairly simple question to answer because I should really already know. Anyway, I was just wondering how exactly a battery is recharged in an AC circuit. In particular, I was looking at an electric car's rechargable battery. I know when you plug the car in the...
29. ### Intermediate Mechanics Practice Test Problem

Hello everyone. The following is from the practice test given to me for my intermediate mechanics final. I'm, at the moment, completely lost on what to do. If you have even just a few ideas of what I should be doing they would be apperciated. If you want to give a detailed solution it would...