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Homework Help: Simple collision between rocks in outer space. Webassign

  1. Feb 13, 2015 #1
    1. The problem statement, all variables and given/known data

    Two rocks collide in outer space. Before the collision, one rock had mass 14 kg and velocity ‹4450, −2950, 2250› m/s. The other rock had mass 7 kg and velocity ‹−600, 1900,3600› m/s. A 1 kg chunk of the first rock breaks off and sticks to the second rock. After the collision the 13 kg rock has velocity ‹1450, 300, 1900› m/s. After the collision, what is the velocity of the other rock, whose mass is now 8 kg?

    2. Relevant equations
    This is an introductory college level course. We were talking about the law of conservation of momentum, but I keep getting the problem wrong when I try to set the two total momentums equal to each other. I keep getting the sample Webassign problem wrong too. I've been working on this for an hour with no luck.

    3. The attempt at a solution

    14*<4450,-2950,2250>+7*<-600,1900,3600>=8*final velocity of other rock+13*<1450,300,1900>

    <62300,-41300,31500>+<-4200,13300,25200>=8*final velocity of other rock+<18850,3900,24700>

    <58100,-2800,56700>-<18850,3900,24700>=8*final velocity of other rock

    <39250,-6700,32000>/8=final velocity of other rock

    <4906.25,-837.5,4000> m/s =final velocity of other rock

    This isn't the correct answer. :( I tried rounding to 3 significant figures as well, and it's still not the right answer.
  2. jcsd
  3. Feb 13, 2015 #2


    User Avatar

    Staff: Mentor

    Check the number of zeros.
  4. Feb 13, 2015 #3
    Oh, thank you! *cries for joy and does a little dance* I don't know why I kept missing that! I got my problem right. It's a bigger relief to know that I actually do understand the concept. :)
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