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Homework Help: What was wrong with my calculation?

  1. Oct 18, 2009 #1
    1. The problem statement, all variables and given/known data

    a 8kg object moving east at 15m/s at a frictionless horizontal surface collides with 10kg object that is initially at rest. After collision, the 8kg object moves south at 4m/s.
    a)what is the velocity of 10kg object after collision?
    b)what percentage of the initial kinetic energy is lost in the collision?
    The answer is a)12.4 m/s at 14.9 angle N of E b)KE lost=7.2%

    2. Relevant equations

    %KE lost = (KE1 - KE2)/(KE1)*100
    KE1 = 1/2*m1*v1**2
    KE2 = 1/2*m1*v2**2 + 1/2*m2*v3**2
    Someone gave me this equation. But I cannot understand lots of symbols such as ** here.
    Note:v3 is velocity of 10kg object after collision, which is 12.4m/s.

    3. The attempt at a solution

    I already solved a) and I was solving b) now. I pugged my value in the equation above assuming that ** means squared. And my answer was 7.4!!!!!!
    how could this can happen? I tried again and again and 7.4 kept came out instead of 7.2
    Does my textbook is wrong of that equation is wrong? If the equation is wrong, pleas tell my the fixed form.
  2. jcsd
  3. Oct 19, 2009 #2


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    Homework Helper

    Yes, I make it 7.4 or 7.5 also.
  4. Oct 19, 2009 #3
    whew! I thought I made a huge mistake in the middle! I must tell my prof that the answer 7.2 is wrong.
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