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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?

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?
3. 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?
4. 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...
5. 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...
6. 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
7. 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...
8. 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...
9. 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...
10. 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...
11. 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...
12. Velocity in Lorentz Transformations

I'm reviewing for exams and don't understand when to use which Lorentz velocity equation to use. one goes v'=(v-u)/(1-vu/c^2) and the second v=(v'+u)/(1+v'u/c^2)
13. Nearest Distance between 2 boats

Ship A is 10 km due west of ship B. Ship A is heading directly north at a speed of 30km/hr, whilst ship B is heading in a direction 60 degress west of norht at a speed of 20km/hr. a) Deteremine the magnitude and direction of the velocity of ship B relative to ship A. b) What will be their...