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3 simple questions

  1. Dec 7, 2003 #1
    1. you are in a rocket ship travelling away from the Earth. Your mother, who is on Earth sends you a message using light with a frequency nu. Should you adjust your receiver to be sensitive to a frequency greater than, the same as, or less than nu?

    Solution: I'm travelling away from Earth so I should adjust my receiver to a frequency greater than or the same as nu.

    2.The force of gravity acting on an object on Earth is almost the same as the force of gravity acting on a space shuttle in orbit. However, the astronauts in an orbiting space shuttle float around the cabin weightlessly. Why?

    Solution: The gravity that is acting on the astronauts is not strong enough so they float weightlessly in cabin.

    3. A photon has an energy of E=5x10^(-19) J. An electron has a momentum p=9.5x10^(-25)kg.m/s. Do these particles have the same wavelength and energy? For an electron, the de Broglie relation is wavelength=h/p and its nonrelativistic energy is E=p^2/(2m). Note: Planck's constant h=6.626x10^(-34) J.s, mass of an electron m=9.1x10^(-31) kg, speed of light
    c=3x10^8 m/s.

    Solution: I have no clue....

    Thanks for helping me...

  2. jcsd
  3. Dec 7, 2003 #2
    First question:

    You are correct(Doppler Effect)

    Second question:

    The question states that the force of gravity is almost the same in the shuttle as it is on earth, so your answer is incorrect. You need to think about what weigthlessness is and you should be thinking about the idea of freefall.

    Third question:

    You are given all that you need to solve this problem, at least the energy part. You are given the momentum of the electron and the equation for that relates the momentum of the electron to its energy. For the wavelength part: You can find the de Broglie wavelength of the electron, so all you need to do is figure out how a photon's energy is related to its wavelength (or frequency since you know that c=f*lambda, where lambda is the wavelength, and f is the frequency)

    good luck,
  4. Dec 8, 2003 #3


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    Norman said you were correct and I see his reasoning but, in my opinion, you haven't answered the question. You were given three options and you answer is two of them!
  5. Dec 8, 2003 #4

    Doc Al

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    Staff: Mentor

    Regarding the rocket ship question:
    Norman didn't supply any reasoning. :smile: But you are right, suzy did give two answers--- but neither is correct. (The receiver is moving away from the source.)
  6. Dec 8, 2003 #5
  7. Dec 8, 2003 #6


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    Regarding Question 2:

    Actually, people can also float inside airplanes that are much closer to the ground. Perhaps there is a better explanation?
  8. Dec 8, 2003 #7
    'Actually, people can also float inside airplanes that are much closer to the ground. Perhaps there is a better explanation?'

    That would be when the plane is accelerating towards Earth faster than 9.81 ms^-2 and this can only be done for short periods of time. The effect is that you are still accelerating towards Earth but as the object you are in is as well, you feel weightless.
  9. Dec 8, 2003 #8
    Thank you for helping me out. ^.^

    1. Solution : Due to a Doppler Effect and the receiver is moving away from the source so the frequency must be greater than or the same as source.

    2. Solution: I'm not sure about this one. I understand that if shuttle falls to the Earth faster 9.8 ms^s, the astronauts will be weightless. But the question is when shuttle is orbiting in space how can astronauts fly wieghtlessly?

    3. Solution: I still don't understand this at all.....help help....

    Suzy ^.^
  10. Dec 9, 2003 #9

    Doc Al

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    Have you been reading this thread? This answer is (doubly) incorrect.
    The apparent weightlessness of astronauts in the shuttle is due to the fact that they (and the shuttle) are in free fall. This can happen in outer space or it can happen right outside your window. The astronauts still have weight (gravity didn't go away!) but they don't feel that weight because nothing is pushing against them. Stand on the roof of a building; you can feel it push against you, holding you up. Step off the building, and you will be "weightless" for a brief period (ignoring the air rushing by). Same is true in an airplane that turns off its engines and starts to plummet. The reason that "weightlessness" is associated with things in orbit is that orbiting bodies are continuously free falling---so there is plenty of opportunity to experience the apparent weightlessness.
  11. Dec 9, 2003 #10
    Question the third:

    Energy of the photon is 5x10^(-19) J

    Energy of electron = it's momentum squared over twice it's mass, therefore E = (9.5x10^(-25))^2 / 2x9.1x10^(-31) = 4.96x10^(-19) J which is as near as damn it to the energy of the photon.

    Energy of a photon, E = hf, where c = fw, w is the wavelength, therefore E = hc / w and w = hc / E
    so the wavelength of the photon is 397.8nm

    The wavelength and momentum of a particle is related by wp = h, so
    w = h / p = 0.698nm

    Conclusion - energies are the same, wavelengths aren't

    Correct me if I'm wrong please
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