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Homework Help: Finding impact speed of a high altitude free fall

  1. Mar 9, 2014 #1
    If we neglect the effects of air resistance, the impact speed of a body of mass m released from rest at an altitude of 2 x 10^8 m above the surface of Earth (mass of earth = M = 5.99 x 10^24 kg, radius of earth = 6.37 x 10^6 m)

    It's a multiple choice and I think I have it, but I just want to make sure because we just scraped over the surface of gravitation.

    I used conservation of mechanical energy where ΔU = -GMm/r and ΔK = (1/2)mv^2
    Setting them equal and opposite, m cancels and we get:

    v^2 = 2GM/r, where r is the radius of earth plus the altitude. I get that the answer is 1968 m/s. Am I doing this correct? I only ask because my answer would then be "None of these is correct" for the multiple choice.
  2. jcsd
  3. Mar 9, 2014 #2
    I just realized that the change in potential energy radius would be the altitude minus the radius of earth...but it still comes out to about 2000 m/s which is still not an answer.
  4. Mar 9, 2014 #3
    I used a different equation for gravitational potential energy: U = -(Gmm/R)(R + h)^-1 where h is the altitude and R is the radius of earth, and found an answer that is in the multiple choice: 1.10 x 10^4 m/s
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