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1. Combinations possible when choosing 4 or 5 team members from

Homework Statement How many combinations of people are there if you choose 4 or 5 from a group of 10? Homework Equations Relies on binomials The Attempt at a Solution binomial (10,4) = binomial (10,6) = 210 But when choosing 5 the answer is binomial (10,5) / 2 = 126 Why do I need to divide by 2?
2. Speed of sound in turbulent air?

My thinking started with wind gusts. I was more thinking if one is bed and the neighbor is noisy could a fan be used to deflect the sound waves around one' s head. I didn't mean the turbulent air would change the speed of sound in the still air, I meant relative to the still air. I don't...
3. Speed of sound in turbulent air?

Could I used turbulent air to deflect a sound wave then? As in my image.
4. Speed of sound in turbulent air?

If I turn on a standing fan in my room how does the speed of sound change in the fast moving air compared to the still air around it? Would it be relatively faster, slower, or does it depend on the direction in which the fan is blowing?
5. Parabola incoming ray not parallel to axis

I don't understand this statement. In my parabola it shines the light rays back from whence they came.
6. Parabola incoming ray not parallel to axis

Assuming the parabolic walls are long enough won't a parabola give you the same result, but only for rays parallel to the central axis. All rays parallel to the central axis will reflect through the focal point, continue through the focal point and hit the opposite parabola wall, and they will...
7. Parabola incoming ray not parallel to axis

Is there a time limit on editing previous posts? Or at least to mark that I've realized I've written something that doesn't make sense This should be changed to, "all light in front of the parabola that is parallel to the central axis will go through or has originated from the focal point of...
8. Parabola incoming ray not parallel to axis

What program did you use to make this? I made my own attempt on paper. I think the answer I was looking for is, all light parallel to the axis goes through the focus, even if the light source is off axis. Yes it's obvious but it didn't make sense till I drew it.
9. Parabola incoming ray not parallel to axis

I can do that when I get home, but then is my ray drawing for the spherical mirror correct, where off axis light sources end up parallel to the central axis?
10. Parabola incoming ray not parallel to axis

It's actually a concave spherical mirror, I didn't know how to sketch a parabola, so I thought it was a good enough approximation. Edit: i see that was a mistake
11. Parabola incoming ray not parallel to axis

If I trace the ray back and out through the focus then the angle of incidence and reflection aren't equal. Which must mean there is something very wrong with my parabola sketch, but I can't see it.
12. Parabola incoming ray not parallel to axis

I used the focal point to determine the normal at each of the points where my source ray reflected of the parabola wall. And the results are, as can be seen in my rough and ready image in yellow, if it hits the parabola walls twice it ends up parallel to the central axis. I almost didn't believe...
13. Parabola incoming ray not parallel to axis

Yeah, I was hoping the focal point would provide a short cut as finding the normal the parabola wall isn't as quick as the normal to a plane surface.
14. Parabola incoming ray not parallel to axis

Would I do it by having the angle of incidence equal to the angle of reflection at each point of the parabola wall?
15. Parabola incoming ray not parallel to axis

Yeah sorry I should have been more clear. The incoming ray reflects of the parabola wall through the focal point then reflects of the parabola wall on the opposite side of the focal point back to the light source. In this case what happens to rays shining into the parabola not parallel to the...
16. Parabola incoming ray not parallel to axis

If incoming light to a parabola is parallel to the central axis the light is reflected through the focal point and the back to its source. What about light coming in not parallel to the central axis. What path do these rays take?
17. TNT Overpressure and Impulse Calculation

After looking online and checking papers I`m not closer to get a useful result. The thing is I find some many different equations and they all give totally different answers. A wall is 3 m high and infinintly long. An explosion of 25 kg TNT is done at a distance of 10 m on the ground. What...
18. Relativity Video Check

Thank you I've put your comment up on the video. When I finished it, I did start wondering how length contraction plays a role. I haven't really heard much about Gaussian units but I will give it a go.
19. Relativity Video Check

I made a relativity video for my father as he's been asking a long time. His question is basically how can light always travel at the same speed no matter where you are. So to answer it and since he's far away I made this video If someone could check it and let me know if it's wrong or...
20. Single bar in Cauchy Schwarz

Thanks I was totally stuck, as in all the notes the professor used double bars.
21. Single bar in Cauchy Schwarz

I need to write out the proof for the Cauchy-Schwarz equation from quantum computing. I'm stuck on the first step as I don't understand what the single bars on the first term in the equation. Double bars is length but single?? http://en.wikipedia.org/wiki/Cauchy%E2%80%93Schwarz_inequality
22. Holography homework problem

I'm really having trouble understanding how holograms work in a good enough way to explain it. I know it captures an interefernce pattern of two incoming waves in the form of a complicated diffraction grating. When the reference beam shines on to the plate the original object is created. But...
23. Understanding the uncertainty principle

The way I look at it is if you're on boat and there are waves with long wavelengths relative to the boat, you don't feel them so much, perhaps a gentle rocking, but if they are the size of the boat or smaller you would crash into them. Note: All the waves have equal amplitude.
24. Understanding the uncertainty principle

I've been trying to understand the uncertainty principle and so far the Wikipedia definition has been very good explainig position and momentum relationship, but the aperture size explanation below is beyond me. Any help? "If a large aperture is used for the microscope, the electron's...
25. How to bring the fun back?

I just finished my 2nd year doing physics at university and the courses have really driven the fun out of it. Just learning equations and procedures, as a result my performance has slipped. It's the first day of summer and I want get back into Physics. I read the how many hours to study...
26. Analysing Op Amp Circuit with a Photo diode

Thank you skeptic2. I just came across the word bypass capacitors that must be it.
27. Analysing Op Amp Circuit with a Photo diode

For a lab we built the following circuit. Our photo diode was receiving a blinking signal which our op amp made visible on a DVM. I do not understand the purpose of the capacitors attached to the power rail. So far I understand how negative feed back with the Rf resistor controls the amount...
28. Position operator is it communitative

I was asked to show how the position operator is not communitative in the Shrodinger Wave equation. I thought it was as it is simply mulitplication [x]=integral from negative to positive infinite over f*(x,t) x f(x,t) dx Can anyone help shed some light on this. I may have misunderstood the...
29. What EXACTLY is momentum?

I was trying to explain momentum to a friend and noticed that momentum is the derivative of energy in respect to velocity. Would this be a correct way to think about momentum, the rate of change of energy.
30. Velocity in Lorentz Transformations

hmm ok here's the question two particles move in opposite directions, with one particle at a speed .784c and the other 0.650c as measured by the laboratory. What is the speed of one particle relative to the other. To get the right answer it's the second equation= .95c v=(v'+u)/(1+v'u/c^2)...