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Velocity change

  1. Mar 14, 2007 #1
    I am trying to solve a physic problem involving the velocity of earth and primitive earth. I have solved it using m1v1+m2v2=(m1+m2)v2 for inelastic equation and get a velocity of 29.65 km/s. Which I think is incorrect since the earth's rotational velocity today is 30 km/s. But I can't figure out what I am doing wrong. Any help on how to correctly solve this problem would be greatly appreciated.

    Problem: The leading theory for the origin of the Moon suggests that a large asteroid the size of Mars struck the Earth. Some of the asteroid material and some of the terrestrial material combined to form the Moon. Assume the combined mass today of the Earth and Moon equals the combined mass of the primitive Earth and the asteroid. Also assume that the asteroid struck the Earth at about 10 km/s. Assume the collision was inelastic.

    How much did the Earth's orbital speed change as a result of the collision

    Present Earth -
    Mass = 5.97 x 10^24
    Velocity = 30 km/s

    Mass = .0735 x 10^24
    Velocity = 1 km/s

    Need velocity of primitive earth

    Thank you for any assistance in solving this problem.
  2. jcsd
  3. Mar 14, 2007 #2
    Why do you assume that your answer is wrong? Taking the sun out of the picture and assuming a 2 ball collision, its seems plausible that the earth was moving slower than vs now.
  4. Mar 15, 2007 #3
    I went ahead and put in my answer for the difference in change between primitive earth and present earth in the online website. My answer for the homework is wrong. Plus being a geology major, it just didn't seem right. I would have thought due to the conditions of the earth back then, it would have been rotating faster.

    I didn't take into account the collision of the asteroid at 10 km/s. I am wondering if I need to in order to get the correct answer.
  5. Mar 15, 2007 #4
    for sure I'd stick that in as the question tells you do so. Seee if that helps.
  6. Mar 15, 2007 #5


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    The direction of the impact, and whether any rotational energy was transferred should be taken into account. There's not enough information in the OP to solve the problem.
  7. Mar 15, 2007 #6
    agreed, thats why I think the simplest assumptions possible should be made.
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