# Search results

1. ### Do you think its possible

Perhaps, but it might be easier to find some special superconducting material that has a critical temperature below room temperature and still works at room temperature...let's say because it's super robust against external perturbations. Ultimately it will depend on the mechanism and the...
2. ### What Produces Gravity?

We don't know the ultimate source of gravitation, and the answers to your questions will have several Nobel Prizes attached to them. Regardless of what gravitation is or what produces it, we know very well by now how it works. As many others have mentioned already, GR is our most advanced and...
3. ### Definition of neighborhood

You can think of a neighborhood as a collection of open balls, which is equivalent to many other definitions. For example, with regard to the two definitions you gave, a set containing an open set of x can be that open set of x itself. Remember, all sets contain themselves, by definition! In...
4. ### E versus K diagrams

To your first question, think about the position vectors that constitute the Bravais lattice. Simply repeating those vectors yields the full lattice structure. Well, the wave vectors k are just another way of reproducing the Bravais lattice, only this time in 'momentum space.' The E versus k...
5. ### What ACTUALLY causes a magnetic field to be generated when there is a current?

I might be reading too much into this question, but it seems like the OP was trying to understand the fundamental cause of magnetism. And for the record, to the OP and to everyone else, "that's how nature works" is always a horrible answer because it doesn't tell you anything. You're just...
6. ### Simple trigonometric problem

This is confusing. Are you trying to show that the left-hand side is equal to the right-hand side (which I don't think is true) or are you trying to solve some other problem?
7. ### Size of Force between two Point Charges

You've got the size of the force down. As for the direction: you have opposite charges, which means they attract one another. So your first answer (the negative one) is the one you should use.

For the first part, you know that the velocity is given by the wavelength multiplied by the frequency. Once you have the velocity, you multiply it with the time interval to get the distance. I'm confused about what the question is referring to in the second part.
9. ### Conceptual Problem (Integration)

Well let's start off simple. We know that 2*pi*r is the circumference of a circle. So the circumference of a circle multiplied by a height h means that we now the have the surface area of a cylinder (excluding tops). Do you see that at least? Once we have the surface area, it's just one more...
10. ### Kinetic energy

It's the energy associated with motion, as opposed to potential energy, which is associated with position. For example, if you're standing still holding a soccer ball in your hands, the ball has potential energy. In other words, it's got the "potential" to do something (to fall to the ground)...
11. ### News What political inclination would you describe yourself as?

I am a radical, social liberal. I support a strong welfare state that helps people assert their identities along with growing global cooperation and interdependence.
12. ### Entropy question

You're partly right. The number of microstates available to the human body far exceeds those available to a simple piece of rock, hence the human body has greater entropy. For that same reason, liquids have greater entropy than solids, which is what you concluded for different reasons.
13. ### Normalising wave functions.

Your result is close but awkward on the first part. Go back and redo the integral. You should get the square root of 2 divided by a, not 1 divided by a. The bounds of integration are 0 to a. Given what you've presented here, the width of the well is a. You have the right idea for the second...
14. ### Force and Orbit Speed

You would need to use the centripetal force equation. If the radius was four times smaller, the force needed to keep the bull in orbit would be << answer deleted by berkeman >> times bigger.
15. ### Dealing with addition of cosntant to wave equation? Spherical Harmonics

Shankar's Principle of Quantum Mechanics has a similar question, and that question asks for the possible L(z) angular momenta and their probabilities. That's what I assumed this person was trying to find, although that's not stated anywhere. Either way, it's all simple. The r^2 part will just...
16. ### Dealing with addition of cosntant to wave equation? Spherical Harmonics

But even if it wants the azimuthal quantum numbers (little L)...that's easy too: just 0 and 2 (given it's a 2nd degree equation)...
17. ### Dealing with addition of cosntant to wave equation? Spherical Harmonics

No the question is about the z part of the angular momentum (implicitly), otherwise you'd be right. But in this case, no phi dependence, and so measured L(z) will always be 0.
18. ### Dealing with addition of cosntant to wave equation? Spherical Harmonics

Everyone is making this too complicated. The allowed angular momentum is just 0. There is no phi dependence in that wavefunction! You just have r and z. You're always going to get 0 with 100% chance.
19. ### Non-Interacting Particles

Ok thanks. So basically: even though they're non-interacting, they're still indistinguishable and they obey the Pauli exclusion principle. Correct?
20. ### What happens when

All hell breaks loose.
21. ### Non-Interacting Particles

I just wanted to clarify something conceptual with non-interacting particles. So if you have three identical non-interacting fermions (say electrons), is it true that they are distinguishable (since they're non-interacting)? Or are they indistinguishable since they're still identical?
22. ### How do you get good at statistical physics?

The answer to your question is rather bland but true. You get good at thermodynamics the same way you get good at anything else in life: you practice like there's no tomorrow. You read about it as much as you can, solve as many problems as you can, and ask as many questions as you can to people...
23. ### Why does the charge uniformly distributed on the surface of the sphere only?

The electric field inside a conductor must be zero, so there can be no charge inside the sphere. The remaining charges array themselves on the surface until there is no field inside the sphere, and the only way that can work is if they are concentrated uniformly on that surface.
24. ### Basic momentum and magnitude questions!

It's just divided by two to get a simpler constant in front of the t^2.
25. ### A Nuclear Fission Problem

You need to calculate the rest mass energies of the reactants and the products and find the difference.
26. ### Question on forces and energy

Yes...but the net work is only equal to the change in kinetic energy. What your book is saying (spring's potential energy) has nothing to do with what you're saying. For a conservative force (ie. only a position-dependent force) like Hooke's law, the force will equal the negative gradient of the...
27. ### Question on forces and energy

U = \frac{1}2kx^2 is the potential energy of the spring. You're talking about the kinetic energy.
28. ### Basic momentum and magnitude questions!

The first part is wrong. You need to use the following formula: Work = Force * Distance So it should just be 354/47, giving you 7.5 meters. I can't tell from what you've written if you're right on part 2 or not, so here's how you do it. You need to set the kinetic energy of the bullet...

Yes.
30. ### Does physics support the notion of eternity?

Relax, tiger. The person who started the thread inquired about "the notion of eternity" and "infinite time." This is a physics forum, not a Catholic seminary.
31. ### Ball throw force problem

The answer is no. The force exerted by your hand was necessary to cause a change in motion, but not necessary for motion itself. This point was the fundamental difference between Newtonian and Aristotelian mechanics.
32. ### Frozen pond friction problem

Please at least attempt a solution next time. I'll help you out anyway because I'm bored. The basic strategy behind solving this problem centers on conservation of momentum. Before we get to that, however, we need to find out the mass of the student. We know the student's weight, W, and from...
33. ### Need Help on Physics Question. Basic Physics

The first objective is to determine the car's deceleration. After that, we use kinematics to calculate the car's initial velocity and compare it to the speed limit. The frictional force acting against the car's motion can be written as: F_f = u_kF_N The normal force is mg, so the...
34. ### Is there life in the universe, and if so has it visited Earth?

Is there life in the universe? I suspect so, but obviously I don't know. Has it visited Earth? I would say no, but obviously I don't know the answer here either. Some of the conversations in this thread remind me of theological debates: a lot of passion for no results. The ultimate...
35. ### Rotational Energy Pulley Problem

Ok this is a basic Atwood machine problem. There are many ways to solve it. I'll do it using the Lagrangian formulation since it's asking for energy methods, but you can do it through standard Newtonian mechanics as well (the two formalisms are physically equivalent). I'll assume that the mass...
36. ### Amplitude Squared Proportional to Intensity?

It would be appropriate to use it if you're given an equation for some EM wave. Then the constant in front squared would represent the intensity of the wave.
37. ### What is Hilbert Space?

Here goes nothing... Hilbert space is a vector space. A vector space is an algebraic structure that contains objects called vectors (in quantum mechanics, these become the wavefunctions), which display the properties of vector addition and scalar multiplication. An algebraic structure...
38. ### Does physics support the notion of eternity?

Most physicists would probably say your question is too philosophical to merit a serious scientific response (the proverbial "what exactly do you mean?" right). However, even after all the chemical and physical reactions have taken place and there is nothing left in this universe but radiation...
39. ### Conservation of energy - velocity problem

This discussion is getting too cluttered when you're already on the right path. The only mistake you made here is that you got the height wrong. It should be 26.7 - 18 = 8.7 meters. Remember, you want the height difference between your original point and your final point (so if radius is 9, the...
40. ### Physics Angular Momentum Question

This question seems confusing. Are you saying something to the effect of...the Earth can't spin too fast because its intrinsic angular momentum would have to equal its orbital angular momentum?
41. ### How long does a ball with a mass m take to hit the ground?

Wait this is weird...is this a double post or something? I already answered this question at this other thread created by the same user: https://www.physicsforums.com/showthread.php?t=359872"
42. ### Projectile F=ma help

Seems like it's right.
43. ### Kenetric Energys, verolcity and mass of a falling object?

For Part i, use the equation y = (1/2)gt^2, where y is the height and t is the time. Solve for t. You should get 1.43 seconds. To find out how fast it's moving, use the equation v = sqrt(2gh), where v is the speed and h is the height. You should get 14 meters per second. For part ii, if it...
44. ### 2d kinematics problem

The total velocity can be written as components of the velocity in the x direction and in the y direction: V(total) = sqrt(V(x)^2 + V(y)^2) where V(x) = V(0)cos(theta) and V(y) = V(0)sin(theta) In the x direction, the object travels the following distance (let's call it D(x), which is...
45. ### 2d kinematics problem

You will need the range equation: R = v^2 * sin(2*theta) / g. Knowing theta and R (the range), solve for v. The time in the air is given by t = 2*v*sin(theta)/g. You know v and theta, so solve for t. If you need me to derive these equations, let me know.
46. ### Newtons 3rd Law

But you said the system was frictionless.
47. ### Newtons 3rd Law

Here's where I'm getting confused. What is mu?
48. ### Newtons 3rd Law

This seems to be an Atwood machine problem, although I don't understand the last part of the problem statement. You will want to set up the equations of motion for each mass. For mass m(1): F1 = m(1)*a = T -m(1)*g = 0 For m(2): F2 = m(2)*a = m(2)*g - T = 0 To solve for tension...
49. ### What is energy

Energy is the ability to do work. Work is the application of a force (push or pull) over a certain distance, so an equivalent definition for energy would say that energy is the ability to apply a force over a distance. Energy is primarily measured in joules, which is kilograms times (meters...
50. ### Can you me with Newtons 3rd Law

Do you have a specific example? I'd hate to just vapidly repeat the definition of the 3rd law, which you can easily find anywhere and everywhere...