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Homework Help: Escape velocity problem

  1. May 6, 2015 #1
    1. Problem
    A rocket has landed on Planet X, which has half the radius of Earth. An astronaut onboard the rocket weighs twice as much on Planet X as on Earth. If the escape velocity for the rocket taking off from Earth is v , then its escape velocity on Planet X is

    a) 2 v
    b) (√2)v
    c) v
    d) v/2
    e) v/4

    The answer is C.

    I reasoned out this problem mathematically, but what is the conceptual reasoning behind this?
  2. jcsd
  3. May 6, 2015 #2


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    I would say this is a mathematical question. To see it more clearly, you could try to derive a formula for escape velocity in terms of the radius ##R## and the surface gravity ##g## of a planet.
  4. May 7, 2015 #3


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    Conceptually you are supposed to quickly approach this from the other end: ##{\tfrac 1 2}mv_{esc}^2## is the kinetic energy needed to overcome the gravitational potential energy. I expect you know how to write the latter as ##GM_{planet}\over R_{planet}##. To see how this ratio relates to idem earth you need Mplanet/Mearth. That follows from the factor 2 in g and the expression for g PeroK is asking for
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