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Homework Help: Cannon shot on airless planet with x gravity

  1. Oct 22, 2008 #1
    I've been struggling with this problem for over two hours...PLEASE HELP!

    1. The problem statement, all variables and given/known data
    A huge cannon is assembled on an airless planet. The planet has a radius of 5.0 \times 10^{6}\;{\rm m} and a mass of 2.6 \times 10^{24}\;{\rm kg}. The cannon fires a projectile straight up at 3800 m/s.

    An observation satellite orbits the planet at a height of 700 km. What is the projectile's speed as it passes the satellite?

    2. Relevant equations
    1/2mv22 - GMm/r+y2 = 1/2mv12- GMm/R

    3. The attempt at a solution

    I'm trying to use that equation and solve for y2. I keep coming up with a negative number.

    v2= sqroot[v12 - (2GM/R) - (2GM/R+y2)]

    you can see that when u subtract, you will get a negative under the root.... im going to go crazy. any help is appriciated.
  2. jcsd
  3. Oct 22, 2008 #2


    User Avatar
    Homework Helper

    Hi Mugen112,

    I don't believe this equation is correct. When you moved the potential energy term from the left side to the right its sign should have become positive.

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