Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook