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Explosive Force- Skateboard

  1. Oct 22, 2014 #1
    1. The problem statement, all variables and given/known data
    Dan is gliding on his skateboard at 3.00m/s . He suddenly jumps backward off the skateboard, kicking the skateboard forward at 6.00m/s (as measured by an observer on the ground). Dan's mass is 40.0kg and the skateboard's mass is 5.00kg . How fast is Dan going as his feet hit the ground?

    2. Relevant equations
    vf-vi=vexplosion*ln(mf/mi)

    3. The attempt at a solution
    vi=3 m/s
    mi=45 kg
    mf of dan=40 kg
    I used 6 m/s as vex
    when I plugged the above in the equation to find vf I got 2.29 which was marked wrong. Is it wrong to use 6m/s for Vex?
     
  2. jcsd
  3. Oct 22, 2014 #2
    The correct answer is 2.63 m/s, but I'm still confused on the process to get the answer
     
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