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Homework Help: Gravitation problem about throwing an object upward

  1. Nov 15, 2011 #1
    1. The problem statement, all variables and given/known data
    Standing on the surface of a small spherical moon whose radius is 6.00*104 m and whose mass is 7.50*1018 kg, an astronaut throws a rock of mass 2.05 kg straight upward with an initial speed 38.5 m/s. (This moon is too small to have an atmosphere.) What maximum height above the surface of the moon will the rock reach?

    2. Relevant equations


    3. The attempt at a solution
    I calculated g, then used the kinematic equation Δy = (vf2 - vi2) / 2a, but it didn't work! Did I use the wrong equations?
  2. jcsd
  3. Nov 15, 2011 #2


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    Staff: Mentor

    Yup. The kinematic equations involving g that we use near the surface of the Earth are valid when g remains essentially constant over the trajectory of the projectile. This holds because the relative change in the radial distance from the center of the Earth is negligible, and g = G*M/r.

    For your moon, the radius is relatively small and the upward launch speed will take the projectile a distance that is not negligible with respect to that radius.

    You would be better to use a conservation of energy approach, using the appropriate expression for the gravitational potential energy as a function of radial distance.
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