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Homework Help: Sped of launchin rocket

  1. Apr 17, 2008 #1
    1. The problem statement, all variables and given/known data
    A rocket is launched straight up from the earth's surface at a speed of 15,000 m/s. What is its speed when it is very far away from earth?


    2. Relevant equations
    K=.5mv^2
    U=-GMm/R


    3. The attempt at a solution
    Is this correct: I used conservation of energy and assumed very far was where U=0

    (.5)(m)(15000m/s)^2 + -[(6.67*10^-11)(5.98*10^24)m]/[6.37*10^6m)=.5mv^2
    v=9988 m/s

    Also could someone explain to me why potential energy decreases with increasing height in this case as opposed to increasing U by lifting an object above the ground on earth.

    Thanks
     
  2. jcsd
  3. Apr 17, 2008 #2

    alphysicist

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    Homework Helper

    Your equation looks okay to me.


    It doesn't; the potential energy is a negative number that get closer to zero with increasing height (i.e., distance away from the earth), which means it is increasing.
     
  4. Apr 17, 2008 #3
    Oh yeah. I see exactly what you are saying. Thanks!
     
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