1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: A 1600 kg object is initially at rest 400 km above the earth’s surface.

  1. Jun 13, 2012 #1
    1. The problem statement, all variables and given/known data

    A 1600 kg object is initially at rest 400 km above the earth’s surface. The object falls straight
    down and generates 8.3x10^8J of heat energy while descending to an altitude of 100 km.
    What is the velocity of the object at this altitude?

    2. Relevant equations

    Ek = (mv^2)/2
    Ep = -GMm/r
    Mass of Earth = 5.98x10^24 kg
    Radius of Earth = 6380000 m
    Force of Gravity = GMm/r^2

    3. The attempt at a solution

    Once I got to the final step I had to find the root of a negative, which is not possible, so I must have made an error somewhere else, but I can't seem to work out where.
    Last edited: Jun 13, 2012
  2. jcsd
  3. Jun 13, 2012 #2
    Your solution is correct, but the mistake lies in 1/r2 - 1/r1, this equals 6x10^-9, not 7.37 x 10^-12.
  4. Jun 13, 2012 #3
  5. Jun 13, 2012 #4
    Oh, god, I knew I'd've made a stupid mistake, I forgot to convert km to m. Ugh, stupid, but at least I got it now, thanks for the help :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook