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Thread: really complicated problems...

  1. #1
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    really complicated problems...

    1. Imagine you are observing a spacecraft moving in a circular orbit of radius 100,000 km around a distant planet. You happen to be located in the plane of the spacecraft's orbit. You find that the spacecraft's radio signal varies periodically in wavelength between 2.99964 m and 3.00036 m. Assuming that the radio is broadcasting normally, at a constant wavelength, what is the mass of the planet.
    I know that the formula to find the velocity of an object is [(apparent wavelength/true wavelength)-1]speed of light, but how do you use this info to find the mass of the planet? #-o

    Thanks,
    Brady Yoon

  2. #2
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    Re: really complicated problems...

    Quote Originally Posted by Brady Yoon
    1. Imagine you are observing a spacecraft moving in a circular orbit of radius 100,000 km around a distant planet. You happen to be located in the plane of the spacecraft's orbit. You find that the spacecraft's radio signal varies periodically in wavelength between 2.99964 m and 3.00036 m. Assuming that the radio is broadcasting normally, at a constant wavelength, what is the mass of the planet.
    I know that the formula to find the velocity of an object is [(apparent wavelength/true wavelength)-1]speed of light, but how do you use this info to find the mass of the planet? #-o

    Thanks,
    Brady Yoon
    The centripetal force that causes circular motion is the gravitational force.

    mv^2/r = GMm/r^2

    m is the spacecraft's mass and drops out. G is newton's gravitation constant. You have v and r - solve for M.

  3. #3
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    swansont - You shouldn't make it so easy, since that was probably a homework problem being posted. [-X

    I would have said, the key words are mass and velocity. 8)

  4. #4
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    How does...

    V^2 = GM/r

    look?

    This is true only for circular orbits, which is a given in this case.


    [From the topic title, I was afraid it was a girl friend issue, so was I relieved ]

    [edit - Gee I'm slow on posting. There were no posts when I started]
    We know time flies, we just can't see its wings.

  5. #5
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    [From the topic title, I was afraid it was a girl friend issue, so was I relieved ]
    Yeah, the topic is very misleading.

    swansont - You shouldn't make it so easy, since that was probably a homework problem being posted.
    Physics homework in 8th grade? No, I was reading an astronomy book for fun and I got stuck.

  6. #6
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    [From the topic title, I was afraid it was a girl friend issue, so was I relieved ]
    Yeah, the topic is very misleading.

    swansont - You shouldn't make it so easy, since that was probably a homework problem being posted.
    Physics homework in 8th grade? No, I was reading an astronomy book for fun and I got stuck. Thanks for the help.

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