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  1. K

    VAWT, wind induction factor

    I'm trying to model a vertical axis wind turbine with the double-multiple streamtube model (with Matlab), where wind speed is dampened by an induction factor so that the output is different for upwind and downwind parts of rotation. The trouble is that for the downwind part, the induction...
  2. K

    Convective heat transfer coefficients

    Hi. I'm trying to make a simple model of a condensation based countercurrent heat exchanger where liquid water and steam flows are separated by a conducting wall. I formed the equations as Q = \frac{T_{steam}-T_{water}}{\frac{1}{Ah_{water}}\frac{d}{Ak_{wall}}\frac{1}{Ah_{steam}}} = L\dot{m}...
  3. K

    How does electricity power devices?

    Hmm. Alright, thanks for the answers everybody.
  4. K

    How does electricity power devices?

    Thanks for the answer, but that really wasn't my question. I was trying to find a general answer, why does any electric device work. What good are mobile electrons in devices which aren't meant for generating radiation, like computers or food mixers? Somehow the electrons manage to make things...
  5. K

    How does electricity power devices?

    I've been wondering how or why electricity actually works. Resistance heaters and light bulbs just turn the kinetic energy of electrons into heat, but what about more complex devices? For example, why does a microwave oven do something useful when electrons flow in the circuitry?
  6. K

    2 Question involving momentum and collisions

    1) You can solve easily solve the momentum of the snowball. Then you need to solve its y component with the help of sine. Because momentum is conserved, the net y-component for the whole system must be zero, so the sled gets an equal but opposite increase in momentum (towards south). Of course...
  7. K

    Potential Energy Car Downhill Question

    What exactly did you do here? It looks like you took square roots of some of the numbers separately, which really isn't allowed. The first equation is correct and you can solve for V simply by diving both sides with 550kg and then taking the square root. You should note however, that all the...
  8. K

    How far the peg is driven into the ground due to impact?

    Wouldn't it be simple to use kinetic energy. The peck has some kinetic energy and the ground does negative work to the peck with a force of 1kN. Just find when the kinetic energy equals the work done by the ground.
  9. K

    Schools University & Physics

    I believe undergradute means Bachelor's degree and graduate stands for Master's degree. Don't know for certain, since I'm not from USA either. In here between a master's and a doctorate there's licensiate (or some such).
  10. K

    Cooling trouble

    That differs a bit from the answer we were shown, but I guess it does just the same thing, you just used Boltzman's constant instead of R. In our version we had C_p=\frac{7}{2}R . I'm not really sure when I should use 5/2 and when 7/2, since both are valid for C_p. t=Q/P where...
  11. K

    Cooling trouble

    Air conditioner cools the air in a room with a power of 8.4*10^6J/h. Volume of the room is 62.5m^3. At first the air is at the temperature of 30C and air pressure is 1atm, which stays a constant. How long does it take to cool the room to 25C? Note that the air shrinks while cooling down and...
  12. K

    Ideal gas problems

    There are some problems which have me completely stumped. 1. A balloon has a volume of 10.5dm^3. Its mass is 8.5g and inside is helium at a pressure of 1.05atm. Atmospheric pressure is 1.00atm and both the helium and the outside air is 25.0C (degrees Celcius). Define the tension in the...
  13. K

    Hello everyone, Angles

    Maybe it's not too late to do some creative censorship. .)
  14. K

    Addition of Forces

    But once you find the mass, you can calculate the weight. The net force in the x-direction causes the acceleration given. You know the relation between force, mass and acceleration, so you can solve the mass.
  15. K

    Who's using here?

    I would use Mastering Physics, but my professor hasn't activated the thingy and put up any exercises. I tried it last year though. Seemed quite useful, though typing all the more special characters was a bit of a chore. Don't have a clear memory anymore...
  16. K

    Dazed and Confused

    What Recon meant is that: 35 mm = 35 X 1 mm = 35 X 0.001 m = 0.035 m One zero too many. .) Here's a table with the values of different prefixes so you can do the conversions:,,sid44_gci499008,00.html Dimensional analysis is used to...
  17. K

    Kinematics - Acceleration and deceleration

    2) How about just starting from the beginning? You have known times and accelerations, so you can calculate the velocities at the end of every part. Current velocity is the sum of the starting velocity and acceleration times time. You just need to do the same thing three times. 3) You need to...
  18. K

    Problem with fluid flow.

    Yes, I ignored the variations in v, because some other user advised me to do so (but then deleted his post). I must say that I didn't understand where you were getting at with those characteristic times and velocities. What are they? I've never seen those equations you presented, I don't...
  19. K

    Problem with fluid flow.

    Okay, I ignored it and got a time of 5.91s. Seems kinda fast, but the volymetric flow looks to be consistent with another problem with the same canister. I guess that's it then, thanks. How about that pool with the hatch? In my book (Young and Freedman's University Physics) there's a similar...
  20. K

    Problem with fluid flow.

    Yes, I understand that, but I have know idea how to apply integrals to problems like these. I have other similar problems which I can't solve because of this. One of them is about a pool. At the side of a pool there's a hatch (dimensions a and b), which is hinged from the bottom and which's...
  21. K

    Problem with fluid flow.

    I have a sylindrically shaped canister (open from the top) with a height of 25.0cm and a radius of 5.00cm. At the bottom there is a hole with an area of 1.50cm^2 spraying out water. How long does it take to empty the canister? Subscripts t and b stand for top and bottom and h is the height...
  22. K

    Stuck on physics problem

    2) Do note that the time the first stone takes to fall down is t while with the second stone it's t-2.3s. This should make forming the equation easier.
  23. K

    Trouble with this equation

    "I end up with R = SQUARE ROOT P2G/3" ----- You made a mistake while dividing. You should end up with r^2 = \frac {3P} {2Gp^2}
  24. K

    Tossing Your Lunch

    You know Henrietta's speed and how long she's been walking (6s+2,77s) when the lunch is at ground level. Now you just need to know how fast the the lunch needs to move horizontally to cross that same distance in 2,77s (the time it takes for it to hit the ground [Henrietta] and thus stop moving).
  25. K

    Can't explain to my friend properly

    Regarding the first question: The way I see it, when the car gets on the lorry it's no longer moving on the same surface (can't find the word I'm thinking of). At first they're both on the road with speeds of, let's say, 60 and 65mph. Now when the smaller car is on the lorry, it would move at...