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Homework Help: Gravitational Potential and Kinetic Energy

  1. Nov 27, 2011 #1
    1. The problem statement, all variables and given/known data

    A rocket is propelled vertically upward from the earth which has a radius R0. After
    its fuel has become exhausted, the rocket reaches its highest point at a height of 2R0
    above the surface of the earth and then falls vertically down back to the earth. With
    what speed does the rocket strike the surface of the earth? Air resistance is negligible.
    Express in terms of R0 and g at the surface of the earth. Show clearly your reasoning.

    2. Relevant equations

    U=-GMm/R KE=1/2mV2

    3. The attempt at a solution

    So I did everything accordingly, I ΔU + ΔK = ΔE which is 0. So -ΔU = ΔK. But my answer includes mass which the question does not want, what am i doing wrong? Shall I use another formula-relation ?
  2. jcsd
  3. Nov 27, 2011 #2

    Doc Al

    User Avatar

    Staff: Mentor

    I assume your answer includes the mass of the earth? If so, you can get rid of it by expressing things in terms of g and R0. (What's an expression for g?)
  4. Nov 27, 2011 #3
    g=Gm/r^2 . Ok thanks! I confused g with G.
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