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Homework Help: Help with Satellites and Grav.

  1. May 8, 2007 #1
    1. The problem statement, all variables and given/known data
    The planet T is a planet of mass M and radius R, and very thin atmosphere (no air resistance). A rail gun has been mounted on the surface of T at the North Pole. A projectile of mass m is fired form the rail gun with an unknown speed vo at an unknown angle θ with respect to the local horizontal. The projectile is observed to rise to a maximum height above the surface of ¼ R. At this maximum height the projectile has a speed of 75.0 m/s. If M=1.5 x 10^20 (such that GM=1.0 x 1010 Nm2/kg) and R=200km, find vo in m/s. and the launch angle θ.

    2. Relevant equations

    3. The attempt at a solution
    I know that F=ma, and Gravitational F=(GMm)\R^2 so ma=(GMm)\R^2 , a in rotational is a=(v^2)\r , so I would set m((v^2)\R)=(GMm)\R^2 . Yet is v in this equation the one they want? In finding the θ once I found the v would there be components in the x and y direction? Can I find it that way? Thank you very much for the help.
  2. jcsd
  3. May 8, 2007 #2


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    The problem doesn't say that the projectile is an a stable [circular] orbit at this point, so that means that it may not obey circular motion. I would say that you would be expected to use conservation of energy here. It would be useful to note that at maximum height vy=0.
  4. May 8, 2007 #3
    Can see what you are saying about consevation of energy, but how will that help me with finding vo
  5. May 8, 2007 #4


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    Hootenanny, I don't think that it is possible here to work with a y axis. By the time the projectile has reached the maximum height, it will be at a point that may be so far from the original point that the y axis has changed quite a bit compared to the initial y axis, if you know what I mean.

    I think one must use conservation of energy (with [itex] - \frac{G m M }{ r} [/itex] of course ) but one must also use angular momentum to answer this question.

    Just a thought.
    Last edited: May 8, 2007
  6. May 8, 2007 #5


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    I think your right nrqed, I've just reread the OP and with a maximum of height of R/4, my method isn't valid. Good catch, hopefully the OP will read this before he/she starts work on it. My apologies newtonistheman :redface:, time to get some sleep I think...:zzz:
  7. May 9, 2007 #6
    How should I go about finding the angle?
  8. May 9, 2007 #7
    Try this way

    1. Conservation of energy: potential+kinetic energy conserved

    ===> relation of v0 and vmax==>v0

    2. Conservation of angular momentum (due to central force)

    ===> relation of v0cos(theta) and vmax==>theta
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