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Homework Help: Help with a few questions please

  1. Mar 19, 2004 #1
    1. What experiment is used to find the Resultant of 2 vectors?

    2. Which wave phenomenon is used to distinguish between transverse waves and longitudinal waves?

    3. Give the equation that defines temperature on the celsius scale.

    4. What is the shortest wavelength of sound that can be haeard in air by a person given that the freq response of the ear ranges from 20Hz to 20kHz?

    5. A tin whistle which acts like an open pipe has a fundemental freq of 256Hz. What is the freq of the second harmonic?

    6. The following reaction represents pair production :
    gamma ray = positron + electron, calculate the min freq of the gamma ray photon required for this reaction to occur.

    There is one other question i was thinking of, Photons have no mass, but when light is shone on a crookes radiometer the vanes rotate? Is this because of the energy that photons have, E=hf (Energy = plancks constant multiplied by the freq).
    Last edited: Mar 19, 2004
  2. jcsd
  3. Mar 19, 2004 #2
    1) No experiment needed here, just simple geometry.

    2) Not sure what you mean here. There are p-waves and s-waves. S-waves cannot travel through a liquid or a gas.

    3) Well, pure water freezes at 0 degrees C and boils at 100 degrees C at standard atmospheric pressure.
    You can convert to Kelvin by adding 273.15 onto the value for C.
    You can convert to F by multiplying the value for C by 1.8 and then adding 32.

    4) Since λ = c/f and c=330 ms-1 The shortest wavelength of sound detectable to the human ear would be 0.0165 m.

    5) If a stationary wave can be set up at 256 Hz, then the second harmonic should be at 512 Hz.

    6) The mass of an electron (and a positron) is 9.11 * 10-31 Kg and the speed of light is 3.00 * 108 ms-1. Therefore plugging numbers into E = mc2 you should get the energy of the photon as 1.64 * 10-13 J. Then using E = hf the frequency comes out to be 2.47*1020Hz. Seems a little high but I checked the numbers and I can't spot any glaring errors.

    Edit: I missed your last question.

    No. Einsteins equation E = mc2 is actually a lot more general than that. The equation is actually E2 = p2c2 + m2c4.
    From this it is possible to extract this bit E = pc where p is momentum. Therefore it is possible to find the momentum of a photon for a given energy since c is constant.
    Last edited: Mar 19, 2004
  4. Mar 19, 2004 #3


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    Could the answer to # 2 be polarization? As far as I know, longitudinal waves are not polarized, whereas transverse waves can be--in certain circumstances, anyway.
  5. Mar 19, 2004 #4


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    I heard once that the radiometer would not rotate for any reasonably-attainable flux of light if there were a perfect vacuum in the glass bulb. Supposedly the effect depends on small amounts of air in the bulb being heated differentially on the dark side of the vanes vs. on the shiny side. I would be interested to hear from anyone on whether this is really the case.
  6. Mar 19, 2004 #5
    In ordinary light bulbs there is Argon gas at low pressure. I was asked a physics olympiad question about why this gas was there. I got it absolutely completely wrong as what I wrote was based on pure quess work.

    It turns out that the low pressure inert gas is there to stop a layer of tungstun ions forming on the inside of the bulb (tungston being the filament in the bulb).
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