Physics question about Spiderman 2 Train Scene

Hi, we've recently been set a task by our physics teacher to try and calculate the Young's Modulus of Spiderman's webs in the train stopping scene of Spiderman 2. Doing research on the mass of the carriage + passengers, calculating forces from that and also using things like the velocity, acceleration/deceleration, length and extension of the webs, number of webs, time taken to stop and distance travelled. So far I have ΔL of the webs to be 600m, the unstretched length of each web to be 16m approx cross-sectional area per web to be 3.14x10^-4 m^2, mass including passengers of the 6 carriages to be 164,637.4kg, final velocity as 0m/s, initial velocity (at time he fires first web) as 35.75m/s and the time between firing first web and coming to a stop as 30 seconds. Also the deceleration as 1.192m/s^2 (or -1.192, I forget if I need the minus or not). Also there are 16 webs in total, assumed to be identical.





I'm guessing I may need the SUVAT motion equations? Definitely equations for tensile stress and strain, and the basic force and speed equations. Obviously I may be wrong which is why I'm asking for some input x)





I haven't attempted much of it yet apart from working out almost every value I can, I need help working out how you would use deceleration to work out the force on the webs as they're horizontal and I'm used to just weights hanging off vertical supports but I would assume it's more or less similar? I'm also not sure how to get the stress and strain with there being 16 webs in total I'm just a little stuck but I would appreciate some expertise here with helping me solve this problem. I would guess the answer to be a little unrealistic, too, based on some values to be approximate and the fact its a Spiderman movie. Thanks :)

Order of upper-division physics classes?

I'm not exactly sure of the intended order of the upper-division physics classes because they aren't ordered one after another.



My current background is all As in: Calc1-3, ODEs, Linear Algebra, lower division mechanics and E&M, and another physics course that didn't transfer from my previous college(covered fluids, sound waves, electromagnetic waves, thermodynamics, and entropy)



I have not had:



General Physics 3 - "Examines sound waves, electromagnetic waves, and geometrical and physical optics. Introduces modern physics, including discovery of the electron, the photon, wave-particle duality, the Bohr model of H-atom, the Schrödinger equation, quantum numbers, the Pauli principle and periodic table, and lasers."



This course isn't offered until spring. I've had the sound waves and electromagnetic wave aspect of it from the physics course that didn't transfer and could self-study the other stuff if it's important for my fall schedule. However, it's not listed as a prerequisite for any of my fall classes.





Fall schedule:



Survey of Partial Differential Equations (Prereqs: Calc 3, ODEs) - "Surveys elementary differential equations of physics; separation of variables and superposition of solutions; orthogonal functions and Fourier series. Introduces boundary value problems, Fourier and Laplace transforms."



General Physics 4 (Prereqs: None listed) - "Examines thermodynamics, including temperature, zeroth law, thermal expansion, specific heat, first law, second law, entropy, third law, kinetic theory, Brownian motion, and the ideal gas. Also explores special relativity, including historical background, Lorentz transformations, length contraction, time dilation, invariance of the laws of physics, relativistic dynamics and kinematics, and paradoxes."



Pretty much all of the thermodynamics covered in this course I've already had.



Intermediate Mechanics (Prereqs: Calc 3, Freshman mechanics) - "

Vectors, Newtonian mechanics: rectilinear motion of a particle, general motion of a particle in three dimensions, oscillations, Hamilton's variational principle: derivation of Lagrange's equations and Hamilton's equations with simple applications , equivalence to Newtonian dynamics, forces of constraint and the Lagrange multiplier method, generalized forces, noninertial reference systems, gravitation and central forces."



Electricity and Magnetism (Prereqs: Calc 3, ODEs, Freshman E&M - "Examines vector calculus, Gauss' law, scalar and vector potentials, Laplace and Poisson's equations, dielectrics, electrostatic and magnetostatic fields, Ampere's law, Faraday's law, and Maxwell's equations."



I'm mainly concered about the E&M course. I have the prerequisites but is it normally taken this early? The suggested sequence (I think it's outdated because sometimes it doesn't make sense) at my university is QM before E&M although I can't figure out why.



The other 2 physics classes seem fine, I'm just wondering when people usually take E&M? If it's completely fine I may add an english course but I was advised to keep the first semester of upper-division courses light.

Invariant Vectors

Hey PF!



I am trying to understand what is meant when we say a vector is invariant, which I believe is independent of a coordinate system. I have already read a PF post here: http://ift.tt/1vptXX4.



I'm looking at DH's post, and this makes a lot of sense!



However, I have read the following, which I am trying to interpret. Please read this and help me out, if you can:



Consider the single point velocity stress tensor, ##v_i v_j## where ##v_i## is the ##i##th component of velocity. First rotate the coordinate system 90 degrees around the ##x_1## axis so the old ##x_3## axis becomes the new ##x′_2## axis and the old negative ##x_2## axis becomes the new ##x′_3## axis. It is easy to see ##v′_2 v′_3## in the new coordinate system must be equal to ##-v_2 v_3## in the old. But isotropy [don't worry about interpreting this] requires that the form of ##u_i u_j## be independent of coordinate system. This clearly is possible only if ##u_2 u_3 = 0##.



Thanks!!

Plane of incidence

How to define the plane of incidence for normal incidence of a plane polarized wave?

Is the reflection coefficient defined by ordinary, extraordinary, or the combination of both waves?



Thanks for the help!

I want to live in Europe

It seems like everything is more fun over there!



USA is getting boring T.T



Need to graduate so I can get out of this country

Child dies in hot car, father charged with murder

So, plenty of us have probably heard about the recent hot car death that will probably become a hugely controversial media circus that everyone will forget about in a few months. I don't know whether the death was intentional or not, but I hope the court makes the correct decision either way.



I don't want to kick this off with a discussion of whether or not the parents should be punished. Instead, I want to consider realistic ways to avoid this sort of thing. Sure, some middle-school student came up with a knee-jerk "solution" that would work in theory, but the fact is that no one at all would have the patience to deal with that device every day.



Cars are getting smarter, and I see no reason why a "smart" approach to this isn't feasible. My idea can be described by the following algorithm:



Precondition: Key is removed from ignition.



1) Check temperature.



Temperature > THRESHOLD ?



YES: Go to step 2.

NO: Go to step 3.



2) Check for (noninsect) life in car.



The detector is a black box, and might not be cheap. But, this sort of problem has been solved for more difficult situations than this.



Was anything living found?



YES: Activate some sort of alarm, perhaps PANIC mode and a radio signal. Remain in this step until manually disabled.

NO: Proceed to step 3.



3) Wait X seconds and return to step 1.



I believe that this is a perfectly feasible system, and it (or some equivalent solution) should find its way into all modern vehicles. What do you think?

Two rotating masses attached by a spring

1. The problem statement, all variables and given/known data



Two pucks of mass m slide freely on a horizontal plane. They are connected by a

spring (constant k and negligible un-stretched length) and set in circular motion

with angular momentum L. The pucks are given a small, simultaneous radial

poke. What is the frequency of subsequent radial oscillations?



2. Relevant equations



Lagrangian? -> Equations of motion?



3. The attempt at a solution

I thought of setting up the Lagrangian of the system and finding the Equations of Motion and then some how apply the small radial pertabation and from that the radial frequency should just pop out. My Lagrangian is

L=1/2m(r1'^2+r2'^2)+1/2m((r1*θ1')^2+(r2*θ2')^2)-1/2k(r1-r2)^2. Since the system is set in circular motion all θ' are constant such that θ'=ω for both theta. With that said the equations of Motions are

-m*r1''+m*w^2*r1-k(r1-r2)=0

&

-mr2''+m*w^2*r2+k(r1-r2)=0

Now how apply the small radial perturbation? is r1-r2<<r1 or r2?

(Note that all time derivatives are denoted by a ', i.e. v(t)=x'(t))

Light Absorption and Transmission

Hello all, I had a question out of curiosity. Say you have a green piece of paper. This paper is green because it absorbs the wavelength of lights corresponding to the other colors of the visible spectrum and reflects (transmits) green light, thus appearing green. If one were to shine a laser that is also green (the same green as the paper), would the paper heat up or would nothing happen? I would imagine it would heat up if it were a laser that was not green as the paper would absorb the light, but what if the laser is green?

Tensors: switching between mixed and contravariant components

I'm working on the electromagnetic stress-energy tensor and I've found this in a book by Landau-Lifshitz:



[itex]

T^{i}_{k} = -\frac{1}{4\pi} \frac{\partial A_{\ell}}{\partial x^{i}} F^{k\ell}+\frac{1}{16\pi}\delta^{k}_{i} F_{\ell m} F^{\ell m}

[/itex]



Becomes:



[itex]

T^{ik} = -\frac{1}{4\pi} \frac{\partial A^{\ell}}{\partial x_{i}} F^{k}_{\ell}+\frac{1}{16\pi}g^{ik} F_{\ell m} F^{\ell m}

[/itex]



I was wondering how this work? [itex]F^{ik}[/itex] is the electromagnetic field tensor, [itex]A_{\ell}[/itex] is the potential of the field.

Turbulence Intensity decrease when Reynolds Number increase

1. The problem statement, all variables and given/known data

I studied a open circuit subsonic wind tunnel. I measuring the turbulence intensity. in many paper says that when Reynold number increases is more turbulent that flow. its meaning that turbulence intensity is higher.

2. Relevant equations

In my experiment, why turbulence intensity is decrease when Reynolds number increases??



3. The attempt at a solution

Speed of a generator?

1. The question is "what speed will the generator need to be driven to give an output of 240V D.C?" the generator is constructed using one pole pair having a sq surface of 6cm x 6cm and produces a flux of 18mWb. It has 4 commutator segments.







2. Possibly F = (P * N) / 120? im not sure







3. F = (P * N) / 120 = (2*3000(dont know the number of turns this was a guess)?)/120 = 50

Einstein's Equivalence Principle and Dark Matter

Einstein's Equivalence Principle postulates that gravitational mass is equivalent to its inertial mass. When you read his theory he is only talking about matter that had, at that time, been observed. He states, "As long as we restrict ourselves to purely mechanical processes in the realm where Newton's mechanics holds sway, we are certain of the equivalence of the systems K and K'. But this view of ours will not have any deeper significance unless the systems K and K' are equivalent with respect to all physical processes, that is, unless the laws of nature with respect to K are in entire agreement with those with respect to K'. " In other words, if Dark Matter is not obedient to Newtons mechanics, which we already know it is not, then the equivalence principle need not be applied to it, according to Einstein himself!



What if Dark Matter's gravitational mass is not equal to its inertial mass? It is an interesting concept. If so, what are the implications? I would be interesting in hearing your ideas and/or your novel concepts regarding the possibility that Dark Matter does not adhere to the Equivalence Principle. What do you think that would that imply?



I am not asking an uneducated question about something that needs to be explained to me. Rather, I would love to see a lively debate and cross exchange of ideas get started here. How would this affect the Big Bang theory? How might it help us to understand an accelerating universe? How might this explain the lack of anti-matter in the universe? etc...

Longitudinal heat transfer material?

hi,

I am running some scientific experiments, and I need a sheet of materials that is extremely good conductor of heat in z axis (through its thickness), I mean it is a good longitudinal heat conductor.

I have been doing some researches and some people told me Ni and Mo foils are good at 90 and 138 W/mk but I was wondering if there is something much better. I have found these graphite Sheets (http://ift.tt/1k7sj7d) with 1500 W/mk but they have such conductivity in xy plane only.

I would be grateful if you can point me to the right direction. that would be great if you can show me where i can buy it too.



thanks

The Force of Gravity Depends on Two Masses

Hello. I don't have a specific homework problem but more of an ideology problem with my brain. According to Newton's law of gravitation (written below), the force of gravity is directly proportional to the masses of both the objects under consideration. Why, then, is the force on a cart that I'm pushing solely dependent on its mass and not mine?





Newton's Law of Universal Gravitation: F = [Gm1m]/r^2



I have some sort of idea for an answer to this question, but none that I am able to verbalize; it's more of an intuition. Can someone much more talented than I please explain this to me in words?



Thank you for any and all help that I receive!

1. The problem statement, all variables and given/known data







2. Relevant equations







3. The attempt at a solution

3-d versus 4-d spacetime curvature

A second SR question that has been on my mind lately is that of hyperbolic nature of Minkowski space. The fact that the invariant interval, or lines of constant delta S trace out a hyperbola according to the equation, ##x^2-(ct)^2=S^2##, is fascinating to me and seems to imply that space-time has a negative curvature.



However, from cosmology I hear that it seems as though the debate over whether the universe has positive, negative, or flat curvature is favoring the "flat" solution according to observations. Does this observation strictly apply only to the 3 dimensional "space only" picture of the universe, or is it a general statement about the nature of spacetime in general?



I remember Penrose addressing this question in the Road to Reality at some point but don't have access to the book to reference it.

Calculating the frequency of an open end pipe?

How can this be done. I know it has to do with the length, but what about the width? Does that have any bearing?

Continuous (non-discrete) Quantum States

I am watching James Binney's QM lectures on iTunes University, and also going through his free textbook. He is a tough teacher, but I love how many misconceptions he points out, and some of the points he makes are very subtle and mind blowing when the lightbulb comes on.



I am confused on this point in his lectures:



He states that ##| \psi \rangle =\int _{ -\infty }^{ \infty }{ dx\psi (x)| x \rangle } ## is an analogy to the discrete state ## | \psi \rangle=\sum _{ i }^{ all }{ a_i | E_i \rangle } ##. Binney uses the lower case psi to describe the complete state that is formed out of the basis static states ##E_i##.



I am sure he is correct, but I need to some baby steps to make a conceptual bridge between the two equations.



Anyone care to flush this out for me?



Thanks,

Chris