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2004-May-03, 07:50 PM
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,

swansont
2004-May-03, 08:04 PM
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,

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.

SciFi Chick
2004-May-03, 08:06 PM
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)

George
2004-May-03, 08:12 PM
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]

2004-May-05, 09:19 PM
[From the topic title, I was afraid it was a girl friend issue, so was I relieved ]

Yeah, the topic is very misleading. :D

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.