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Nov
17
comment How does a cyclist moves the center of mass of the cycle-cyclist system?
@user: He does steer the front wheel. Try what I said.
Nov
17
comment How does a cyclist moves the center of mass of the cycle-cyclist system?
@user: You don't move your center of mass, you move your point of support. Try balancing anything: a baseball bat, ladder, broom. You do it by moving the point of support. Try riding your motorcycle and keeping the front wheel on a narrow line on the pavement so it can't go left or right. You can't do it for long.
Nov
13
comment In a circuit one light-second long, does it matter where in the circuit the switch is placed?
Can't you arrange the wire so that the inductance is zero? Then doesn't the question make sense?
Nov
7
comment How does inhalation work?
I still have my favorite physical law: Nature abhors a vacuum. It especially seems to apply to computers and software, and even to wallets :)
Nov
6
comment Calculating effective capacitance of a circuit
The problem with just giving the answer is the asker doesn't learn, and neither does anyone else.
Nov
3
comment What is Pressure Energy?
@TimKrul: No fluid is truly incompressible. If it were, the speed of sound in it would be c. Incompressibility only means it is quite stiff, in which case it can have high pressure, but not much pressure energy. i.e. d=0 in your equation.
Nov
1
comment Why is paper (or any tailless) airplane pitch stable?
@RoelSchroeven: We're basically in agreement. As John Denker points out, the term is decalage. The point is, there needs to be an upward pitch moment that increases with speed, counteracting the nose-heaviness. Civilian aircraft are built that way. Fighter jets with fly-by-wire are not. They sacrifice stability for maneuverability, but it takes a computer to fly them.
Oct
28
comment Can a glider maintain a constant forward velocity component?
@Arty: Same question. Why would doubling $v$ divide $D$ by four? I think that negative sign in $D = Av^{-2}$ is wrong, and that ripples through all your math.
Oct
26
comment Can a glider maintain a constant forward velocity component?
By the way, where do you get $D$ as proportional to $v^{-2}$? That would say that if you double the speed, the drag would decrease by a factor of 4. I'm sure that's not what you meant.
Oct
23
comment Heat produced in collision
You know the energy before the collision. You also need to know the energy after. The difference is the heat.
Oct
22
comment Jim Gates' discovery of error-correcting codes in equations of supersymmetry
@ACuriousMind: OK, I just did, but I guess it has to pass inspection. (Basically, it's an African word for a particular type of symbol. I'm sure you've seen lattice diagrams corresponding to Boolean systems, and Gates talks about manipulating these.)
Oct
22
comment Jim Gates' discovery of error-correcting codes in equations of supersymmetry
@ACuriousMind: I put in the tag hoping it would collect questions about this. If that's bad, I will take it out.
Oct
18
comment Bernoulli principle and particle
@Tonylb1: Suppose you take a small segment of the wall facing the fluid. It has a certain area a. It feels a force f, which is nothing but the impact of fluid particles bouncing off of that area of wall. The definition of pressure is force divided by area, f/a. So you can make the area as small as you like, or maybe not so small. The bigger it is, the more force it will feel because it has more area for more molecules to bounce off of. The smaller, the less, but either way, the ratio of force to area, the pressure, stays the same. You don't have to resort to infinitesimals.
Oct
16
comment A quantum particle moving from A to B will take every possible path from A to B at the same time
@StarDrop9: Go to YouTube and search for "water wave motion physics". Here's one result that shows it very well.
Oct
15
comment A quantum particle moving from A to B will take every possible path from A to B at the same time
@StarDrop9: Take 1 photon. Its quantum "reality" is that it is a probability wave, exactly equivalent to its electromagnetic wave. (Other particles also "are" waves - just not electromagnetic.)The energy in that wave at a place and time is the probability that the photon can be observed at that place and time. That wave spreads everywhere it can go, but it must interfere, and where its energy is smallest, the photon can basically not be seen.
Oct
15
comment Water bombing - risks and possibilities
@SamuelWeir: Force is $m dv/dt$. If that rainwater comes all at once, so $dt$ is small, the force will be destructively large.
Oct
7
comment How does wind cause a wind turbine to spin?
A turbine blade is just a wing. Please see how wings work. There are explanations here that are better than anything you will find on this site.
Oct
6
comment How does gas spin the turbine in a jet engine?
@Ethan: Are you asking how a windmill works, or a propeller? It's just a wing that goes around in a circle. Are you asking what a wing is? Start with a greased wedge, supporting a weight. Push sideways on the wedge - the weight goes up. Push down on the weight - the wedge squeezes out sideways. That's what a wing is - push it forward and it pushes air down. Push the air up, the wing slides sideways.
Oct
6
comment How to calculate thrust from opening a pressure vessel in space?
@Crashworks: or she ;)
Oct
6
comment How does gas spin the turbine in a jet engine?
@Ethan: No, the fuel makes the air hot, which makes it big, which makes it blow out the back (because the back is just a big hole). The turbine is just a fancy windmill.