## Search found 709 matches

Sun Jan 23, 2011 11:44 pm UTC
Forum: Science
Topic: Thermodynamics Questions
Replies: 13
Views: 2461

### Re: Thermodynamics Questions

First Problem: You're screwing up either here F(b) = p(density of air)*V*g or here (pick only one): Bouyant force [F(b)] - envelope weight - load = 27.0 N depending on how you want to define bouyant force. Second problem: Just write the volume in each case as cross-sectional area (A) times the relev...
Sun Jan 23, 2011 11:20 pm UTC
Forum: Science
Topic: Privatized Space Travel
Replies: 148
Views: 11162

### Re: Privatized Space Travel

Yes, but you're using current infeasibility to predict continued problems 300 years in the future. I just don't buy that. Is this thread about private space travel in 300 years? If so, then it is impossible to rule out the practicality of any technology that doesn't violate the laws of physics. A m...
Sun Jan 23, 2011 7:22 pm UTC
Forum: Science
Topic: Privatized Space Travel
Replies: 148
Views: 11162

### Re: Privatized Space Travel

You get more than 2/3, because after the 8 hours, you still get some energy Okay, fine. But still 3x the ground area, so 1/3 the efficiency you were talking about before. Who cares about land-efficiency? Even at a population of 30 billion, we won't run out of non-arable land. The important thing he...
Wed Jan 19, 2011 10:06 pm UTC
Forum: School
Topic: Professors shouldn't be allowed to do this
Replies: 29
Views: 5409

### Re: Professors shouldn't be allowed to do this

To people If those questions are effective at separating the students who understand the material from the students that don't, that is the only relevant measure of fairness. From your descriptions, that is the case- so not only should professors be allowed to do this, but they should do it more. Lo...
Mon Jan 17, 2011 6:02 am UTC
Forum: Science
Topic: Microwaves, Stealth Aircraft and the Cold War
Replies: 20
Views: 1902

### Re: Microwaves, Stealth Aircraft and the Cold War

letterX wrote:
thoughtfully wrote:It also had oh-my quantities of altitude, which helped too.

Yes, but it gets beat in altitude by that silly plane named after some band, so we'll focus on the speed (at which it is still the fastest).

Is there a band called MiG-25? Or perhaps X-15?
Mon Jan 17, 2011 5:45 am UTC
Forum: Science
Topic: Either I have missed something obvious...
Replies: 10
Views: 1862

### Re: Either I have missed something obvious...

I don't understand this question. It seems to me the answer should be very straightforward - if the rope is of a fixed length, then an object must swing in a circle centered about the fixed point of the rope. The direction of movement will then always be tangent to this circle. Since we know the pe...
Tue Jan 11, 2011 6:12 pm UTC
Forum: Science
Topic: Using an Abundance of Wasted Energy
Replies: 44
Views: 3960

### Re: Using an Abundance of Wasted Energy

gmalivuk wrote:"Every little bit helps" is a perfectly fine sentiment, as long as you're talking about net benefit and keeping in mind the associated costs.

If "net" means taking opportunity costs and the discount rate into consideration as well as other costs, then yes.
Wed Jan 05, 2011 7:44 pm UTC
Forum: Science
Topic: Using an Abundance of Wasted Energy
Replies: 44
Views: 3960

### Re: Using an Abundance of Wasted Energy

A turbine would slow down the water as it goes down the drain. This would cause a vastly increased chance of clogs and of water backing up into your tub. That's not worth the headache. The storm water turbine might work until a tree tries to go through it.
Mon Jan 03, 2011 10:21 pm UTC
Forum: School
Topic: Thoughts on math homework in college
Replies: 27
Views: 4968

### Re: Thoughts on math homework in college

I assume this is something like calculus or diffeq where problem-solving is emphasized over proofs. I don't see why homework is even graded at all in college classes like that. If it were up to me, homeworks would be assigned, but not collected, and solutions would be provided about a week later. Ma...
Thu Dec 23, 2010 12:03 am UTC
Forum: Science
Topic: Squirting
Replies: 19
Views: 2915

### Re: Squirting

Let's start off with some definitions: Static Pressure: This is the pressure of the water in its own reference frame . Imagine tossing a pressure gauge into the flow, and letting it come to rest relative to the water. The pressure it would measure is the local static pressure of the water. Dynamic P...
Sat Dec 18, 2010 6:00 am UTC
Forum: Science
Topic: Kilogram as a unit of weight
Replies: 51
Views: 5496

### Re: Kilogram as a unit of weight

We measure weight in pounds! Anything else is a direct attack on my culture! The lack of millipounds and kilopounds makes it a unit fit only to weight grain in 18th century England :D What lack? Those may not be common units or anything, but I wouldn't say they don't exist, on account of you just s...
Thu Dec 09, 2010 10:30 pm UTC
Forum: Science
Topic: Misunderstanding tidal forces
Replies: 5
Views: 789

### Re: Misunderstanding tidal forces

You are correct in how differentiation affects units. Remember that G is in units of m^3*kg^-1*s^-2. This will make the units work out. The delta-R is the difference in distance between two points on the object. If delta-R=0, then the two points are actually the same point, and there is obviously no...
Sun Nov 28, 2010 7:30 am UTC
Forum: Science
Topic: Mechanical Hysteresis?
Replies: 2
Views: 1530

### Re: Mechanical Hysteresis?

It's not really due to friction, it's due to the material itself changing. You ask "Shouldn't an extension of three inches exert the same force in all cases (assuming the same materials, sufficiently similar conditions, etc)?" The answer is yes, all else being equal, a certain extension sh...
Mon Nov 22, 2010 8:35 am UTC
Forum: School
Topic: Trying to decide my major. Any help from you guys?
Replies: 10
Views: 2040

### Re: Trying to decide my major. Any help from you guys?

I'm a 4th-year Aerospace Engineering major. As an AE, you'll learn the fundamentals of just about every field that relates to aircraft or spacecraft. It mostly breaks down to four fields: Fluid Mechanics/Thermodynamics All about the physics of fluids. This includes everything from wing theory to com...
Tue Nov 16, 2010 6:55 pm UTC
Forum: Science
Topic: Dynamics Question
Replies: 7
Views: 724

### Re: Dynamics Question

The cable is attached at the horizontal surface in the upper-left corner. It runs down to A and then around the pulley at A, then to pulley B and back to A, where the other end is secured. Its total length is therefore (Distance from horizontal surface to A)+(Two times distance from A to B). As A mo...
Tue Nov 16, 2010 6:51 pm UTC
Forum: Science
Topic: Mechanical Engineering, or, A Crisis in Profession
Replies: 8
Views: 1462

### Re: Mechanical Engineering, or, A Crisis in Profession

If you want to do a specialization that is involved with much experiments, a physics background will be a benefit. Physics undergrads tend to have much more experience in setting up and running experiments than engineering undergrads. How many lab courses do physicists have to take? fyi, E&M is...
Tue Nov 16, 2010 7:55 am UTC
Forum: Science
Topic: Mechanical Engineering, or, A Crisis in Profession
Replies: 8
Views: 1462

### Re: Mechanical Engineering, or, A Crisis in Profession

I would recommend asking some professors who work on earthquake-resistant structures what they think. Here's a couple from my university: http://www.ce.gatech.edu/people/faculty/451/overview http://soliton.ae.gatech.edu/people/jcraig/research.html Send them (and anyone else you find) a brief, courte...
Tue Nov 16, 2010 4:16 am UTC
Forum: Science
Topic: Dynamics Question
Replies: 7
Views: 724

### Re: Dynamics Question

I like how the spring is a completely spurious addition to the problem. The cable could just as easily be replaced with a rigid bar with no change to the fundamental problem. The point of the problem is that the hypotenuse between the two blocks changes as the position of a changes. A rigid bar wou...
Tue Nov 16, 2010 2:58 am UTC
Forum: Science
Topic: Dynamics Question
Replies: 7
Views: 724

### Re: Dynamics Question

iroZn wrote:x = (l/2)cos(θ) - (y/2)cos(θ)

x' = -(y'/2)cos(θ)

[Mythbusters]Well there's your problem.[/Mythbusters]
Theta isn't constant with time.
Mon Nov 15, 2010 11:23 pm UTC
Forum: Science
Topic: Undergraduate Statics Textbook
Replies: 19
Views: 3031

### Re: Undergraduate Statics Textbook

Our structural analysis class (in Aerospace Engineering) covers advanced topics like non-symmetric bending and thin-walled members, because those are common in aerospace structures. It doesn't do anything non-static (that's a separate class) or do anything terribly advanced with trusses or frames.
Mon Nov 15, 2010 5:01 am UTC
Forum: Science
Topic: Undergraduate Statics Textbook
Replies: 19
Views: 3031

### Re: Undergraduate Statics Textbook

I really liked my school's text, by Gere and Goodno. I'm taking a more advanced structures course right now, and I use the linked book as a reference a lot.
Sat Nov 13, 2010 3:02 am UTC
Forum: Science
Topic: Undergraduate Statics Textbook
Replies: 19
Views: 3031

### Re: Undergraduate Statics Textbook

@whereswalden90: I don't think there is such a thing as "advanced statics". The term is used for unmoving, rigid-body problems only, and those are in the end pretty trivial. It's just something engineers have to ingrain completely, the way you do with high school algebra. So you might try...
Mon Nov 08, 2010 6:35 am UTC
Forum: Science
Topic: What do you think the next type of engineering will be?
Replies: 26
Views: 3260

### Re: What do you think the next type of engineering will be?

Saying an entire field doesn't exist because it isn't at some arbitrary level of achievement makes no sense to me, especially when that level of achievement is not in at least the near future. Someone in 1910 might just as easily have said "Aeronautical Engineering won't really be a type of eng...
Mon Nov 08, 2010 2:39 am UTC
Forum: School
Topic: Should I take Intro Chem?
Replies: 6
Views: 1776

### Re: Should I take Intro Chem?

I'm an engineering student in my freshman year of college. I haven't declared my engineering specialization yet, but I have taken calculus and physics in high school and I'm ready to choose. However, my advisor wants me to take intro chem (basically AP Chem) next semester because I don't have AP cr...
Mon Nov 08, 2010 1:32 am UTC
Forum: Science
Topic: What do you think the next type of engineering will be?
Replies: 26
Views: 3260

### Re: What do you think the next type of engineering will be?

I feel that the next 'big' engineering will be the one that will give the most benefits to aviation and motor vehicles generally. Improving the efficiency of existing designs until something else revolutionary comes along that takes off properly... Something like that would receive a lot more fundi...
Fri Nov 05, 2010 4:27 am UTC
Forum: Science
Topic: Light/Heat "Fire" without oxidization?
Replies: 24
Views: 2603

### Re: Light/Heat "Fire" without oxidization?

Just about any explosive.
Sat Oct 30, 2010 7:58 pm UTC
Forum: Science
Topic: Fomrula for compression of water
Replies: 2
Views: 3226

### Re: Fomrula for compression of water

My Google fu has failed me. The only answer I've found is that water (and all liquids) compress, but with absolutely no description of how beyond "it's difficult". I know (ideal) gasses compress under the formula V = k/P. I know that's obviously not true for liquids. So what is the formul...
Mon Oct 25, 2010 8:30 pm UTC
Forum: Science
Topic: Recomend me some chemicals to read about on wikipedia
Replies: 24
Views: 3095

### Re: Recomend me some chemicals to read about on wikipedia

Mon Oct 25, 2010 5:53 am UTC
Forum: Individual XKCD Comic Threads
Topic: 0803: "Airfoil"
Replies: 243
Views: 100559

### Re: 0803: "Airfoil"

Because of weight force, the airfoil injects a constant flow of down-momentum into the air. Where does this momentum end up? Are there some parcels out there where down-momentum is accumulating? We look at a region local to the airfoil. Is down-momentum in the region constantly increasing? Doesn't ...
Sat Oct 23, 2010 10:51 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 0803: "Airfoil"
Replies: 243
Views: 100559

### Re: 0803: "Airfoil"

As I understand it, vector "flux lines" of aerodynamic forces must terminate on vorticity sheets associated with surfaces (as with a venturi,) or must be associated with free regions of vorticity, as with propulsion by vortex-shedding. I think you're thinking of Helmholtz's theorems . If ...
Sun Oct 17, 2010 11:48 pm UTC
Forum: Science
Topic: [PHYSICS] Challenge Problem
Replies: 38
Views: 5440

### Re: [PHYSICS] Challenge Problem

Using Lagrangian Dynamics: L = T - U = 1/2 \lambda (L-y)(\dot{y})^2 - \frac{\lambda g (L-y)^2}{2} The Forces will be: -\frac{\delta L}{\delta y} = 1/2 \lambda \dot{y}^2 - \lambda g (L-y) So the Normal Force is this above without the mg term: N = 1/2 \lambda \dot{y}^2...
Sun Oct 17, 2010 8:47 am UTC
Forum: Science
Topic: [PHYSICS] Challenge Problem
Replies: 38
Views: 5440

### Re: [PHYSICS] Challenge Problem

A rope of mass m and length l is suspended by one end above a scale, with the lower end just barely brushing the scale. The rope is dropped at time t=0. Find the reading of the scale as a function of time.
Sun Oct 17, 2010 7:31 am UTC
Forum: Science
Topic: your thoughts on "number stations"
Replies: 26
Views: 2471

### Re: your thoughts on "number stations"

Izawwlgood wrote:And why jam it? Why not just fill the airwaves with even more clutter and noise?

Isn't that the definition of jamming?
Thu Oct 14, 2010 9:51 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 0803: "Airfoil"
Replies: 243
Views: 100559

### Re: 0803: "Airfoil"

Note well that in a 2D flow diagram the net downwash is exactly zero, the airfoil performs no net work upon the air, and the airfoil flies by pressure difference! 2D diagrams accurately describe ground-effect flight, or a venturi. Unlike with helicopters and hovering rockets, airfoils in 2D diagram...
Wed Oct 13, 2010 6:28 am UTC
Forum: Science
Topic: Gravitational Potential Energy Question
Replies: 12
Views: 1842

### Re: Gravitational Potential Energy Question

If you're doing non-calc-based physics, they should have given you an equation for this situation. If you're in calc-based physics, you should have an equation that relates a force field to potential energy. Set the potential energy an infinite distance from the planet to be equal to zero and go fro...
Tue Oct 12, 2010 8:24 pm UTC
Forum: Science
Topic: Bullet drop
Replies: 25
Views: 2956

### Re: Bullet drop

Thesh wrote:Also, all these cool weapons don't protect us from IEDs, which is the leading killer of troops in both Iraq and Afghanistan.

OT: Here's some cool stuff that will, though:
http://kitup.military.com/2010/10/natur ... flame.html
Mon Oct 11, 2010 10:43 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 0803: "Airfoil"
Replies: 243
Views: 100559

### Re: 0803: "Airfoil"

Nope, instead they explain the flight of infinitely-wide wings. They're a very useful shortcut in calculation, but they offer no explanation. To explore the hidden flaw in infinite-wingspan diagrams, just ask yourself where the other end of the 3rd-law force is located. There's an upward force on t...
Sat Oct 09, 2010 3:36 am UTC
Forum: Individual XKCD Comic Threads
Topic: 0803: "Airfoil"
Replies: 243
Views: 100559

### Re: 0803: "Airfoil"

First, let's examine the case where the air that flows over the top of the wing exactly meets the air that flows over the bottom. This is known as "laminar flow". What is the lift in this situation? That is not what "laminar flow" means. See here for the real definition. Well, c...
Fri Oct 08, 2010 7:59 am UTC
Forum: Individual XKCD Comic Threads
Topic: 0803: "Airfoil"
Replies: 243
Views: 100559

### Re: 0803: "Airfoil"

And then it gets WORSE: It's impossible to understand airplanes by only employing 2D flow diagrams. These diagrams depict an odd sort of "flatland flight" where no work is performed on the air, and an airfoil could ideally coast along forever. While it's true that most 2D flow diagrams yo...
Fri Oct 08, 2010 5:52 am UTC
Forum: Individual XKCD Comic Threads
Topic: 0803: "Airfoil"
Replies: 243
Views: 100559

### Re: 0803: "Airfoil"

The only answer I've ever gotten that I have been satisfied with is that fluid flow is so complex (see Navier-Stokes) that lift isn't fully understood even by those with PhD's in the subject. ... Even in my 300 level fluid dynamics class, it was more or less explained as: "When flow changes di...

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